I hate Statistics.

Anyway.

Two independant events are such that P(A)=0.2, P(B)=0.25, P(AuB)=0.4, P(AnB)=0.05

All I have to figure out is:

Given that exactly one event has occured, find the probablity that A occurs.

I know the formula for given is P(X|Y)=P(XnY)/P(Y) and the answer is 0.2*0.75/0.35

I just want to know where each of the numbers come from. Mainly the 0.75. Thanks.