Answer:
1:5 or you can write 1/5 or 1 to 5
Step-by-step explanation:
I set this up as an inequality,

. If you take the cubed root of 800, you get the lower bound of the side length, which is 9.2. Then I just worked my way up until I hit the first number that put me over a volume of 800. That number is 9.29, because 9.28 cubed is 799.1 (not high enough) and 9.29 cubed is 801.8. Therefore, the bounds of the sides exist within a conjunction:

. That's the best I could come up with to help on that one. Wasn't sure if there was another method you were taught at school. I just used common sense more than any rule.
Vol. of a rect. prism is V = L*W*H.
Here, that vol. is V = (2.5 ft)(1.5 ft)(4 ft) = 15 cubic feet (answer)
Note: volume (not valume).