24% is close to 25% how could you use this fact to check that 50 is a reasonable number for the total number of students in the choir

You didn't specify the total number of students in the choir, but I can tell you this about estimating (especially when doing it mentally):

When you are estimating answers, it is easiest to use large, round numbers such as 25%. It is easy because it is the same as saying 1/4 of something. On the other hand, 24% would be 6/25 (in simplest form), which would be harder to calculate.

6/25 is the correct answer