Explanation:
if you did not leave anything out, then the whole class means the mathematics class of 24 students.
that means
100% = 24
1% = 100%/100 = 24/100 = 0.24
to see how many % the 3 students are, we need to find how many times 1% (0.24) fits into 3 :
3 / 0.24 = 12.5%
the 3 students are 12.5% of all of the students in the class.