# Thread: another probability question, i am doing it wrong as i get 1.5

1. ## another probability question, i am doing it wrong as i get 1.5

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.

2. Originally Posted by SydneyBristow
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.
the required probability =P(f | e)= P( f and e) / P(e)

3. how did you derive that the probability of there being no sisters is 3/14 and being only child is 2/14?