Why can you use fraction multiplication to check the answer to a fraction division problem? how do you use multiplication to check a division problem?

Fraction division is basically multiplication of a reciprocal.

For example (1/4)/(1/8) = (1/4)x(8/1)=2

Multiplication of the answer and the piece that divides (in this case 2 x 1/8), you should get the piece that is being divided (in this case 1/4). If that is correct, then your division is correct.

The density of the cube is 40