Answer:
8 large boxes
12 small boxes
Explanation:
s=small box. l=large box
Volume of small box(s): 8 cubic feet ==> 8s
Volume of large box(L): 21 cubic feet ==> 21L
Total number of boxes: 20
Total volume: 264 cubic feet
s + L = 20
8s + 21L = 264
(s + L = 20)*8 ==> multiply the equation by 8 in order to turn 's' into '8s'
8s + 8L = 160 ==> simplify
8s + 21L = 264
- (8s + 8L = 160)
8s-8s + 21L-8L = 264-160 ==> take the difference of the above equations
13L = 104 ==> simplify
L = 8 ==> divide both sides by 8 ==> 8 large boxes
s + 8 = 20 ==> plugin 8 for L in the equation s + L = 20
s = 12 ==> subtract 8 on both sides ==> 12 small boxes