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Translate birthday counts and find a strict minimum sample size

It may be assumed that the dates of birth in a large population are distributed throughout the year so that the probability of a randomly chosen person’s date of birth being in any particular month may be taken as 112\dfrac1{12}.

(i) Find the probability that, of 6 people chosen at random, exactly 2 will have birthdays in January. marks

(ii) Find the probability that, of 8 people chosen at random, at least one will have a birthday in January. marks

(iii) NN people are chosen at random. Find the least value of NN such that the probability that at least one will have a birthday in January exceeds 0.9. marks

(iv) Find the probability that, of 100 people chosen at random, at least 40 will have birthdays in May, June, July or Agusut. marks

[J84/I/12]

CJC · Review Problems · Question 9

Use the complement for at least one and reverse the inequality when dividing by a negative logarithm.

Read the worked solution