Lets x to be number of students.
1) If each students get 3 ml, then all students get 3x ml.
There was n ml.
(n-3x) ml leftover
n - 3x = 5
2) If each students get 4 ml.
then all students get 4x ml.
n+21 = 4x
3)
n - 3x = 5
- (n+21 = 4x) -------> -n -21=-4x
n - 3x = 5
-n -21= - 4x
-3x-21=5 - 4x
x=5+21
x=26
Answer : 26 students
Check:
x=26, n-3x = 5, n-3*26=5, n=83
n+21 = 4x
83+21=4*26
104 = 104 true