70% of 100 is 70and 60% of 100 is 60. 70% bought lunch and 30% didnt, so that means 79 bought lunch and 30 didn't. onky people who bought lunch left, so the 30% remained. but 60% of the students bought lunch stayed in the cafeteria. 30%+60%=90% . 90% is 90 students, so there are 90 students in the cafeteria