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A and B alternately toss a coin (A first). The probability that A wins (first head) is
A2/3
B1/4
C1/2
D1/3
Answer & Solution
Correct answer: A. 2/3
1. P(A wins on toss 1) = 1/2; on toss 3 = (1/2)^3; on toss 5 = (1/2)^5; ...
2. Sum = (1/2) + (1/8) + (1/32) + ... a geometric series with a = 1/2, r = 1/4.
3. Sum = a/(1 - r) = (1/2) / (3/4).
4. = 2/3.
_Source: ICAI BoS Foundation Paper 3, Ch 15 'Probability', p.12_
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