Your friend has generated two random numbers from $1...10$, independently of each other. What is the probability that both numbers are even given the information that there is an even number among the two numbers?
The solution says $0.4$ but I have no idea how they got that number. My approach is to use Bayes' formula like so:
$A$ is the event of both numbers being even and $B$ is the event of at least one number being even. Then
$$P(A|B) = \frac{P(B|A)P(A)}{P(B)} $$
$$ = \frac{1 \cdot 0.25}{0.75}$$
$$ = \frac 1 3$$