for question 21, we divide 480 seats by 4 = 120 because it is out of 4 units. 3 x 120 = 360 seats is occupied (3 out of 4). 480 - 360 = 120 (unoccupied)

for question 22, just multiply 36 pencils x 28 boxes.

for question 23, when you draw out the model, you will find 4 units. find the answer for one unit, you can get melanie's no. of stamps.

let me know if you need me to explain further! :)