Given
E={1,2,3,4,5....14,15}.... sample space
A={2,4,6,...14} ..... even numbers
B={4,8,12} ............multiples of 4
To find: P(A|B)
We deduce from above that
P(A)=7/15
P(B)=3/15
P(A ∩ B)=3/15 since B is a subset of A.
The conditional probability P(A|B) is the probability of A happening given B has already happened.
So if a number has been selected, and it is known that B has happened, (i.e. multiple of 4), we know that there is a 100% probability A has happened (even).
Mathematically, P(A|B) is defined as P(A ∩ B)/P(B).
So
P(A|B)
=P(A ∩ B)/P(B)
=(3/15)/(3/15)
=1 (as reasoned above).
Answer: P(A|B)=1