In a rectangular solid, there are 6 faces let's say A, B, C, D, E and F from among which we can have 3 pairs such that two faces of each pair will have the same area and will be opposite. For example A and D will be opposite and have the same area.
B and E will be opposite and have the same area.
C and F will be opposite and have the same area.
What the second statement is saying that out of these 3 pair one pair will be of area 40.

Solution:
Obviously each statement alone is not sufficient to answer the question.
Let us check after combining both the statements.
Let the sides of the solid be a, b and c.
So ab =15, bc = 24 and ac = 40.

What we need is abc.
Now (abc)^2 = 15*24*40 = 14400
Or abc is 120.

The correct answer is hence (C).