This delightful problem is taken from a collection of 200 "problem solving cards" designed for eighth grade. It's the hardest problem of that set, but of course you are well beyond eighth grade! Consult the original card here. The problem statement below does not say that the tank is lying on one of its three sides, but it's clear from the picture on the card.

A fuel storage tank is in the shape of an equilateral triangular prism. The prism is 8 feet high and 10 feet long. When the tank is half-full, the depth of the fuel is feet.
Hint: Ask not what's in the tank, ask what isn't.