While if only 35% were seated, that means that 65% were not seated. And since it tells us 91 students were not seated, 91 is the 65% of the total amount of students. So we set up a proportion 91/65 =x /100 . When we cross multiply we get 9100=65x, divide by 65 on both sides and get 140=x. Therefore, there were 140 students in all.
