SALES AND MARKETING Department MATHEMATICS. Combinatorics and probabilities. Tutorials and exercises

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1 SALES AND MARKETING Department MATHEMATICS 2 nd Semester Combinatorics and probabilities Tutorials and exercises Online document : section DUT Maths S2 IUT de Saint-Etienne Département TC J.F.Ferraris Math S2 CombProb TEx Rev2018 page 1 / 9

2 Exercise 1. (Tutorial for lesson page 8) E is the set of the inhabitants of a city ; Card(E) = A is the set of this city s men ; Card(A) = B is the set of this city s retired people ; Card(B) = women are retired. Create, then complete, a contingency table; tell how many men aren t retired; tell how many people are women or retired people. Exercise 2. Given A and B, two subsets of E, simplify the following expressions: A ( A B) B ( A B) ( A B) A ( A B) B ( A B) ( A B) Exercise 3. E = {2, 5, 8, 11, 14, 17, 20}. Let A be the subset of the even numbers of E and B the one of the multiples of 5. 1) Define the complement of A in E. Give its elements. 2) Give the sets A B and A B. What is their union? Exercise 4. In a group of 25 students (including 17 women), 20 passed their exam (including 14 women). 1) Build a contingency table dispatching the information above. 2) How many men passed their exam? What is the name of the corresponding set? Exercise 5. Amongst 350 people interviewed for a survey, 244 own a computer with an Internet access, 287 own a smartphone, but inside this last group, 56 don t have a computer with an Internet access. 1) Organize and fill a contingency table using these data. 2) How many people own a. an Internet access but no smartphone? b. at least one of both? c. only one of both? Exercise 6. After counting the answers from a survey conducted on a sample of 500 people, it appears that 154 of them go to the cinema at least once a month, 228 buy popcorn when they go to movies, and of those who go to the cinema less than once a month, 131 usually buy popcorn. 1) Let s name A the set of people who go to the cinema at least once a month and B the set of people who buy popcorn when they go to movies; build the corresponding contingency table. 2) Answer by naming the adequate subset and justifying the result if not already written in the table: a. Among people who go to the cinema at least once a month, how many buy popcorn? b. How many people go to the cinema less than once a month? c. How many people are elements of A or B? IUT de Saint-Etienne Département TC J.F.Ferraris Math S2 CombProb TEx Rev2018 page 2 / 9

3 Exercise 7. (Tutorial for lesson page 9) * by how many ways can we arrange two objects inside three drawers? * how many numbers composed with four figures only contain the figures 1, 2, 3? * how many words can be written by taking five letters chosen in the set {a, b, e, m, i, r, o}? Exercise 8. (Tutorial for lesson page 10) * how many pairs representative/assistant could have been elected from a group of 25 students? * how many ways can 3 blocks be piled, taking them among 10 blocks of different colours? * how many words can be written by taking five different letters chosen in {a, b, e, m, i, r, o}? Exercise 9. (Tutorial for lesson page 12) * How many couples of representatives could be elected from a group of 25 students? * How many different hands of 8 cards could be given from a deck of 32 playing cards? * How many draws of 6 different integers are possible, taking them between 1 and 49? Exercise 10. (Tutorial for lesson page 12) 1) From a deck of 32 playing cards, how many 8-card hands own exactly 3 spades and 2 hearts? 2) In a company, among 20 women and 20 men, 5 women and 3 men have to be chosen at random. How many possibilities are there? Exercise dice 1.1 A die is being rolled three times. How many possible outcomes? 1.2 Three dice are being rolled at the same time. How many possible outcomes? 2. numbers and letters 2.1 How many phone numbers of eight figures can theoretically exist? 2.2 How many ways can six different integers be chosen among [1 ; 49]? 2.3 How many numbers are composed with three different figures (including 0)? 2.4 How many different lists of 4 letters can be created for the vehicles number plates? 2.5 How many anagrams of the word "MATHS" are there? 2.6 How many words can be created, taking 4 letters from the word "BRACKET"? 2.7 How many 10 notes-long melodies can be written, taking notes among A,B,C,D,E,F,G? 3. arrangements 3.1 How many ways can 5 objects be arranged into 8 boxes? 3.2 Same question, but you can t place more than one object per box. 3.3 Paul drives a team of 5 people. Each month, he evaluates the work of one of them, chosen at random. In a one year period, how many different lists of evaluated people could have been made? 4. playing cards 4.1 How many 8 cards hands from a deck of 32 playing cards? 4.2 How many 5 cards hands from a deck of 52 playing cards? 5. classifications, elections 5.1 How many possible different tiercés at the end of a 12 horses race? 5.2 How many possible different classifications at the end of a 12 horses race? 5.3 How many ways a delegation of 5 people can be chosen from a group of 40? pilots are fighting at a formula 1 race. At the end, the only six first will score different numbers of points. How many possible distributions of points are there? 5.5 How many possible podiums after a race in which 8 runners will compete? 5.6 Fifteen people meet. Everyone gives a handshake to every other, once. How many handshakes? Exercise 12. Let consider a group of 13 women and 8 men. 4 people have to be chosen. 1) How many possibilities are there? 2) How many of them contain exactly one man? 3) 2 men? 3 men? 4 men? no man? IUT de Saint-Etienne Département TC J.F.Ferraris Math S2 CombProb TEx Rev2018 page 3 / 9

4 Exercise 13. Among the hands of 5 cards taken from a deck of 32 playing cards, how many contain: a. the 4 aces? b. a square? c. exactly 3 spades? d. exactly 2 spades and one club? e. at least one king? f. at least two jacks? g. exactly 3 diamonds and one king? Exercise 14. A jar contains balls: 2 white, 3 green, 5 red. 3 balls are to be taken simultaneously from the jar. Among the possible groups of 3 balls, how many contain a. one single colour? b. the three colours? c. two colours? d. at least two colours? Exercise 15. On loto game (former rules), a player has to select 6 different numbers from the set {1, 2, 3,, 49}. Then, the official draw is performed, revealing the 6 winner numbers. 1) Calculate the whole number of possible selections. 2) Among them, how many would contain exactly a. the six winner numbers? b. 5 winner numbers? c. 4 winner numbers? d. 3 winner numbers? e. no winner number? (find two ways ) Exercise 16. 1) Independent questions a. In order to build your team, you have to choose 5 people out of a group of 10 basket players. How many different teams could be made? b. A company has 18 employees and its manager decides to give three awards : best employee, most punctual employee, and less bald employee. How many ways can these awards be given? c. On a chess board (8 8 tiles), how many ways can you put a king, a queen and a tower? 2) 20 chips have been put in a bag, numbered from 1 to 20. The chips from #1 to 10 are white; those from #11 to 16 are green; the last ones are red. You have to draw three chips, at random, simultaneously. a. How many different possible draws are there? b. How many draws can be made with three white chips? c. How many draws would show three different colours? d. How many draws would show three chips of the same colour? e. How many draws would show three even chips of the same colour? Exercise 17. (Tutorial for lesson page 16) A random experiment consists in taking one integer, at random, among {1 ; ; 20}. 1) Determine Card(Ω). 2) Let s name some events: A : "get at least 15" and B : "get an even number". Determine : p(a), p(a), p(b), p(a B), p(a B). Exercise 18. (Tutorial for lesson page 16) A random experiment consists in a simultaneous drawing of 3 letters in our alphabet. 1) Determine Card(Ω) 2) b. We set the following events A: "get 3 consonants", B: " get 2 consonants", C: " get 1 consonant" and D: "get 3 vowels" a. Are they mutually exclusive? b. Do they represent a partition of Ω? c. Calculate their cardinal numbers and then their probabilities (writing four significant figures). Finally, check the sum of their cardinal numbers and the sum of their probabilities. IUT de Saint-Etienne Département TC J.F.Ferraris Math S2 CombProb TEx Rev2018 page 4 / 9

5 Exercise 19. (Tutorial for lesson page 16) Random experiment: roll two dice and add both results. 1) What are the different possible sums? 2) Are they equally likely? 3) Build a sample space of equally likely outcomes. 4) We set the following events: A: "the sum equals 10" and B: "the sum is at least 10". Determine p(a) ; p(a) ; p(b). Exercise 20. (Tutorial for lesson page 17) E = {1, 2, 3,, 10}, A: "even numbers of E", B :"multiples of 3 in E" Venn diagram: probabilistic choice tree: E A 8 10 A 2;4;6;8; B 7 9 contingency table: A A Inside, cardinal numbers of the B = Card(B) corresponding intersections must be B = Card(B) placed, subtotals ("marginal = Card(A) = Card(A) = Card(E) frequencies") and the overall total 1) Complete the contingency table. 2) Let s choose a number between 1 and 10, at random, not looking at it. a. What is the probability it would be even? b. What is the probability it would be a multiple of 3? 3) Let s choose a number between 1 and 10, at random, not looking at it, but we re told it s a multiple of 3. a. What is the probability it would be even? b. What is the result obtained by the corresponding formula? 4) Let s choose a number between 1 and 10, at random, not looking at it, but we re told it s even. a. What is the probability it would be a multiple of 3? b. What is the result obtained by the corresponding formula? Exercise 21. (Tutorial for lesson page 17) A laboratory has developed a breathalyzer. A reliability test has been done on this product, with a testpopulation on which it's been stated that 2% exceed 0.5 g/l (event E) and so are out of law. Everyone exhales into the breathalyzer; the event P refers to a positive result given by this device. The reliability test has given the following results: - 95% of people who really exceed 0.5 g/l got a positive result by the breathalyzer; - 96% of people who don't exceed 0.5 g/l got a negative result by the breathalyzer. What is then your probability of really exceeding 0.5 g/l, given that your result is positive? Exercise 22. (Tutorial for lesson page 17) 1) Taking back exercise 21: are E and P independent? 2) Taking back exercise 17: are A and B independent? Exercise dice are rolled together. 1) How many different outcomes are possible? 2) Calculate the probabilities of the following events: a. "get a triple 6" b. "get a triple" c. "get a 421" d. "get at least one 4" e. "get a sum of 10" 3) a. Are B and D mutually exclusive? b. Are C and E mutually exclusive? A 1;3;5;7;9 IUT de Saint-Etienne Département TC J.F.Ferraris Math S2 CombProb TEx Rev2018 page 5 / 9 6B B 2;4;8;10 B3;9 B 1;5;7

6 Exercise balls lay in a jar: 7 white, 9 red, 2 green. Three balls are simultaneously taken out. 1) What is the probability to see the three colours in your hand? 2) What is the probability to see only one colour in your hand? 3) a. Which third event makes a partition of Ω with both former ones? b. Deduce its probability. 4) Calculate the probability to get no white ball or no green ball. Exercise 25. A bag contains 20 coins: n are black and the 20-n others are white. 2 coins have to be picked up together. 1) Express (with n) the probabilities of the following events: a. A: One black and one white. b. B: two black c. C: two white 2) Check that the global probability equals 1. 3) Determine, by solving an equation, the values of n such that p(c) > 0.5. Exercise 26. A bank found that 2% of the checks issued by its clients aren't correctly worded (correctly worded : event W). 97% of correctly worded checks are correctly entered by the agent into the bank's data base (event E). When it's not correctly worded, the agent is able to correct the mistakes 5 times out of 100. A check has to be entered into data base. calculate the probabilities of the following events : 1) The agent doesn t enter the check correctly. 2) The check has been correctly worded, given that the agent entered it correctly 3) The check has not been correctly worded, given that the agent didn t enter it correctly Exercise % of people in a city are employees. Among them, 8 people on 10 use their car every day, whereas 30 % of the unemployed do (employee: event E; use car every day: event D). We are to select one person in this city, at random. 1) Display the different categories of people in a probabilistic choice tree. 2) a. What is the probability that this random individual be unemployed? b. What is the probability that this person be an employee who uses his/her car every day? c. What is the probability that he/she uses his/her car every day? d. Given that this person drives his/her car every day, probability he/she is an employee? e. Are the events E and D independent? Exercise 28. (Tutorial for lesson page 16) A lottery is held. 100 tickets are to be sold, 1 each. One ticket is a 30 winner, two are 15 winners, and seven would make the buyer win 1. Considering we want to purchase one ticket, X is the random variable of the net gain (the 1 expense taken into account). 1) Give the probability distribution of X. 2) If we re playing this lottery the same way many times, can we expect to be a long-term winner? (begin by an estimate of what would be likely to occur after a thousand attempts) Exercise 29. (Tutorial for lesson page 16) 1) Calculate the expected value and the standard deviation, with the data of exercise 28. Comment. 2) If the possible gains and loss were doubled (in the initial array), what would these parameters become? 3) If the values of X were increased by 0.5, what would these parameters become? Exercise 30. (Tutorial for lesson page 17) From a jar that contains 7 white balls and 3 black balls, let s draw two balls, one after the other and without putting back the first one. We name X 1 the random variable corresponding to 1 point in case the first ball is black and 0 point in case it s white ; we name X 2 the random variable corresponding to 1 point in case the first ball is black and 0 point in case it s white. a. Give p(x 1 = 0) and p(x 1 = 1). b. Give p X1 = 0 (X 2 = 0) and p X1 = 0 (X 2 = 1), then p X1 = 1 (X 2 = 0) and p X1 = 1 (X 2 = 1). IUT de Saint-Etienne Département TC J.F.Ferraris Math S2 CombProb TEx Rev2018 page 6 / 9

7 c. Complete the probabilistic choice tree and then the associated probability table. X 1 X 2 P X2 P X1 1 d. Is the knowledge of both marginal distributions sufficient for the knowledge of the joint distribution? e. Compare p(x 1 = 0) p(x 2 = 0) to p((x 1 = 0) (X 2 = 0)). Are the variables X 1 and X 2 independent? Exercise 31. A game consists of two identical boxes each having 10 chips numbered 1 through 10. The experiment is to pick a chip in each box. 1) a. Describe one of the possible outcomes. b. Explain why the sample space s cardinal number is 100. c. What is the probability of choosing two even numbers? d. Prove that the probability of two different even numbers is ) For one game, you have to spend 1. If you get two different even numbers, you win 1; if you get two identical numbers except 1 and 1, you earn 6; 1 and if you get the double one, you win 50; in all other cases, no gain. The random variable X gives the gain at the end of the game, regardless of the initial 1 bet. a. Give the probability distribution of X. b. Give the expected value of X. c. Can we expect to win money on playing this game a lot? Exercise 32. A bag contains 5 white and 10 black balls. You bet 2 for a 3 balls draw together. Get 3 white makes you earn 100; 2 white: 10; 1 white: 2; 3 black: nothing. The random variable X is your gain at the end of a test, once deduced the bet. 1) Give the probability distribution of X. 2) Give the expected value and the standard deviation of X. 3) If you play a hundred times, what gain is the most likely? Exercise 33. A game consists of a random draw of a letter from the alphabet (which contains 20 consonants and 6 vowels A, E, I, O, U, Y). Each letter is assigned a number according to the following table: A B C D E F G H I J K L M N O P Q R S T U V W X Y Z We'll set the events C: "the letter is a consonant" and M: "its number is at least 17". Part 1 1) Build a probabilistic choice tree (1st level: C and its contrary; 2nd level: M and its contrary) into which the simple, conditional and intersection probabilities will be placed. 2) Given that a vowel has been drawn, what s the probability its number is more than 16? 3) Given that its number is more than 16, what s the probability it s a vowel? 4) Are the events M and C independent? Part 2 The event C M, is awarded a 10 gain and the event C M would make you lose 5; as for the other possibilities: they don t lead to either gain or loss. X is the random variable gain after one draw. 1) Give the probability distribution of X. 2) Give the expected value of X and its meaning. 3) Give the standard deviation of X and its meaning. IUT de Saint-Etienne Département TC J.F.Ferraris Math S2 CombProb TEx Rev2018 page 7 / 9

8 Exercise 34. Two sales agents A and B of a cooperative work in team for two weeks to obtain orders from potential customers. A is responsible for placing new contracts to existing customers and B is responsible for prospecting new customers. Let s name: X A the random variable measuring the number of contracts obtained by A and X B the random variable measuring the number of contracts obtained by B. It s assumed that X A can only take its values in {0 ; 1 ; 2 ; 3} and X B in {0 ; 1}. The joint distribution of X A and X B is given through the following table: X B X A ) a. Determine the margin distributions of X A and of X B. b. Are these variables independent? 2) Let s set a new variable, X total number of obtained contracts, by X = X A + X B. a. Give the probability distribution of X. b. Calculate E(X) and V(X). IUT de Saint-Etienne Département TC J.F.Ferraris Math S2 CombProb TEx Rev2018 page 8 / 9

9 IUT TC MATHEMATICS FORM "COMBINATORICS AND PROBABILITIES" IUT de Saint-Etienne Département TC J.F.Ferraris Math S2 CombProb TEx Rev2018 page 9 / 9