72% of 25 students of a class are good in mathematics. how many are not good in maths in that class?

First you need to calculate how many student is good at math, and that is by knowing how many student forms the 72% of the class total number.
This is by multiplying the total number of the students by the percentage that are good at math.

So 18 students are good at math.
So the rest, which are not good at math, are equal to;

The answer is 7.
Hope this helps!