Question

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

Answer 1

72% of 25=18
25 there are
18 are good at math
7 aren't good at math

Answer 2

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.
$25* \frac{72}{100} =18$
So 18 students are good at math.
So the rest, which are not good at math, are equal to;
$25-18=7$
The answer is 7.
Hope this helps!