Multiple choice general knowledge math & puzzles

Ronald and Michelle have two children. The probability that the first child is a girl, is 50%. The probability that the second child is a girl, is also 50%. Ronald and Michelle tell you that they have a daughter. What is the probability that their other child is also a girl?

  1. 1/9

  2. 2/9

  3. 1/3

  4. 2/3

Reveal answer Fill a bubble to check yourself
C Correct answer
Explanation

The sample space for two children is {BB, BG, GB, GG}. Given they have a daughter, the 'BB' case is eliminated, leaving {BG, GB, GG}. Only one case (GG) has the other child as a girl. Thus, the probability is 1/3.