Subtract 220 from 500 to get the amount that you have for students.
you should have 280 left
then divide that by 7
here's a easy way to do this. If you know that 28/7 is 4, then 280/7 will be the same as 4•10 since 28•10 is 280
4•10=40, which means 40 students can go on this trip.
