You will need 28 vans. 3 students will ride in the van that isn't full.
The ratio is 4:5 girls to boys
28/36 will be your answer which can be simplified into 14/18 which can be simplified again to 7/9 and that's your final answer
$21 because 3$x$7(numbers of days in a week)is $21. I hope this helps :)
