Project ForgeGeneral Aptitude
Quantitative Aptitude
GATE 2017
Medium
A couple has 2 children. The probability that both children are boys if the older one is a boy is
- A1/4
- B1/3
- 1/2
- D1
Explanation
Given specifically that the OLDER child is a boy, the younger child's gender is independent and equally likely to be a boy or girl, so P(both boys | older is a boy) = P(younger is a boy) = 1/2. (This differs from the classic 'at least one is a boy' puzzle, which gives 1/3, because here the identity of which child is a boy is pinned down.)
Source: GATE ME 2017 Session 2 (ME-2) (2017)
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