Question

if 3 students sit on each bench,1 student will be left.if 4 students sit on each bench,1 bench will be left. find the number of students and benches,

Answer 1

there will be 5 benches and 16 students.[verify the by actually doing the sum]

Answer 2

b benches  and s students
3 b = s -1     3 students sitting on b benches.  one student left
4 (b-1) =  s     4 students sitting on b-1 benches. all students sit.
 So    4b -4 = 3 b + 1  =>  b = 5  s = 16