Let n be unknown number, then 1/5 of this number is 1/5n. When you add these two numbers, you get

You know that this sum is equal to 24, so

1. Multiply this equation by 5:

2. Solve it:

Answer: n=20, correct choice is B.
I think maybe 3) would make the most sense because she wants a random sample of students and the other options are specific on the type of students.