Your answer needs to be 8 because 4/5 of 400 = 320 and if you multiply 320 by 2 because 2 chaperones for every 75 and divide 640 by 75 you get 8.5 and you can round that to 8.
So, you first need to take 4/5 of 400 (320). Then, you can count by 75s for a bit
75students; 2chaperones
150; 4
225; 6
300; 8
now, you have 20 students left over...since one chaperone would watch about 37 kids, you can either divide the remaining kids up among the other chaperones, or you can add one more chaperone to watch the 20 kids. i don't know what the teacher wants, so either 8 or 9 chaperones
