You need to use the conditional probability formula:
P(A|B) = P(A∩B) / P(B)
Key:
P(A|B): Probability of A given B
P(A∩B): Probability of A and B
If we say A is the event that the train arrives on time and B is the event that the train leaves on time, then according to the information:
P(B) = 0.4
P(A∩B) = 0.28
Just insert into the formula:
P(A|B) = 0.28 / 0.4 = 0.7
(or 70% as a percentage).
