First find the side lengths of the box by factoring each area:
12: 1, 2, 3, 4, 6, 12
15: 1, 3, 5, 15
20: 1, 2, 4, 5, 10, 20
Find 3 numbers that both have 2 sides have in common and can multiply to make the areas of the faces:
12: 3 * 4
15: 3 * 5
20: 4 * 5
Sides: 3, 4, and 5
Then multiply to get the volume:
3 * 4 * 5 = 60 units^2