Question

In an experiment, three people toss a fair coin one at a time until one of them tosses a head. Determine, for each person, the probability that he or she tosses the first head. Verify that the sum of the three probabilities is 1.

Answer 1

Players probabilities of winning are 4/7 , 2/7, 1/7 which of course sum to 1.

Explanation:

The coin theoretically could give a very large number of tails first so each person's probability is made up of an infinite series.

P(1st person wins) = P(H) + P(TTTH) + P(TTTTTTH) + . . . etc

= 1/2 + (1/2)^4 + (1/2)^7 + (1/2)^10 + . . .

This is a geometric series with first term a = 1/2 and common ratio r = 1/8

Using formula a/(1 - r) this is (1/2)/(7/8) = 4/7

P(2nd person wins) = P(TH) + P(TTTTH) + P(TTTTTTTH)

= (1/2)^2 + (1/2)^5 + (1/2)^8 + . . .

Geometric series with sum (1/4)/(7/8) = 2/7

P(3rd person wins) = P(TTH) + P(TTTTTH) + P(TTTTTTTTH) + . . .

= (1/2)^3 + (1/2)^6 + (1/2)^9 + . . .

Geometric series with sum (1/8)/(7/8) = 1/7

Players probabilities of winning are 4/7 , 2/7, 1/7 which of course sum to 1.

Hope this helped!