suppose for simplicity that the number of children in a family is 1, 2, or 3 with probability 1/3 for each. Little Bobby has no sisters. What is the probability that he is an only child.

I got 1.5 which is CLEARLY wrong.

let E= there are no sisters

F = being an only child

Probably of there being no sisters is 3/14 and being only child is 2/14

P(e/f) = (3/14)/(2/14) = 1.5 which is wrong.