In a class of boys and girls, there are 50 learners, 15 of which are girls. If a learner is picked at random. Find the probability that the learner is either a boy or a prefect, leaving your answer in the simplest form.

Total number of learners = 50
Number of girls = 27
Number of prefects = 15
Number of girls who are prefects = 7

Number of boys = Total learners - Number of girls

So, there are 23 boys.

Number of boys who are prefects = Total prefects - Number of girls who are prefects

So, there are 8 boys who are prefects.

We use the principle of inclusion-exclusion:
$N(\text{Boy or Prefect}) = N(\text{Boy}) + N(\text{Prefect}) - N(\text{Boy and Prefect})$

There are 30 learners who are either a boy or a prefect.

Probability = $\frac{\text{Number of favorable outcomes}}{\text{Total number of outcomes}}$

The probability that the learner is either a boy or a prefect is $\boxed{\frac{3}{5}}$.