Question

in a class if 3 students sit on each bench 1student will be left if 4 students sit on each bench 1 bench will be left find no of students and no of benches in that class

Answer 1

If there 16 students in the class then if 3 students sit on 1 bench. 1 student will be left. Number of bench will be 5. And if 4 students sit on 1 bench then 1 bench is left.

Answer 2

let number of students be "N"
and number of benches be"n"
so 3 students on each benches student left will be 1
3n+1=N
4n-4=N
or
3n-N=-1.....1
4n-N=4.............2
subtracting 1 and 2 we get 
n=5
putting n=5
in in 1
15-N=-1
N=16
so number of student are 16 and number of benches are  5