Question

Helppppp how do u do this????

Answer 1

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