ans is 60 benches

let x=no of benches

y=no of students per bench

given

x*y=600,---------(1)

and given that 10 benches are less hence 2 students are seated more than per bench then

600=(x-10)*(y+2)

600=xy+2x-10y-20------------(2)

since xy=600

then eq(2) is

2x-10y=20

2x-10(600/x)=20

on solving

we get x=60