==> probability/oldest.girl.s <== There are four possibilities: Oldest child Youngest child 1. Girl Girl 2. Girl Boy 3. Boy Girl 4. Boy Boy If your friend says "My oldest child is a girl," he has eliminated cases 3 and 4, and in the remaining cases both are girls 1/2 of the time. If your friend says "At least one of my children is a girl," he has eliminated case 4 only, and in the remaining cases both are girls 1/3 of the time.