What are the chances? There is a class with 20 students.
We pick 4 lucky students who can go to the cinema.
What are the chances? Andrew can go to the cinema, but his best friend John can't go with them.

The probability that Andrew will be picked out is (as for every student) $\frac4{20}=\frac1{5}$. Under the condition that Andrew is picked out the probability that John will not be picked out is $\frac{16}{19}$.
So:$$\Pr(\text{Andrew lucky}\wedge\text{John unlucky})=$$$$\Pr(\text{Andrew lucky})\Pr(\text{John unlucky}\mid\text{Andrew lucky})=\frac15\frac{16}{19}=\frac{16}{95}$$
