Suppose that a class contains 15 boys and 30 girls, and that 10 students are to be selected at random for a special assignment. find the probability that exactly 3 boys will be selected.

Here, I'll use C for combinations.
There are 15 boys and you need 3 so 15 C 3 ways
There are 30 girls and you need 7 so 30 C 7
and there are 45 total and you need 10 so 45 C 10

The answer is [15 C 3 • 30 C 7] over 45 C 10

so do that on your calculator if you need an actual decimal.