Answer C
A rectangular solid has 6 sides and each side has the same dimensions/area as the side opposite to it.

http://www.durangobill.com/IntBrick2.gif (link if the picture isn't working)

Here's a picture to help visualize it. (the numbers in the picture mean nothing, just using it to visualize the shape and how the sides interact)

1. Two adjacent sides have areas 15 and 24

doesn't help us, we need all the dimensions

2. Each of two opposite faces has area 40

doesn't help us on it's own,

but together it tells us that the 3 pairs of sides have areas 15, 24 and 40

now we write up a formula

xy = 15

xz = 24 (x is in both because they're adjacent and share a side)

yz = 40 (the last side is made up of one side of each of the other ones

x=(15/y)

z = (40/y)

xz = 24 so (15/y)(40/y) = 24

600/y^2 = 24

600=24y^2

25 = y^2

y = 5

plug in 5 for y and you get the rest

5x = 15 so x = 3

5z = 40 so z = 8

sides are 3, 5 and 8

volume = 3*5*8 = 120