Answer
$18$ beginner, $15$ intermediate, $15$ expert.
Work Step by Step
Find $37.5\%$ of $48$ (number of beginner trails):
$$48\cdot 37.5 \%=48\cdot \frac{37.5}{100}=48\cdot 0.375=18$$
The rest is:
$$48-18=30$$
Divide rest evenly between two trails (intermediate and expert):
$$30:2=15$$
$18$ beginner, $15$ intermediate, $15$ expert.
