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.