How do I work backwards?
Some problems give you the answer, you have to make up the question.

If given the total length for perimeter of a rectangle.....
Choose one side length and double it.
Take that number off the total length.
Divide that number by two.
Now what do you get?

If given the total area of a rectangle.....
Choose a side length.
Divide the total area by that side length.
Now what do you get?

If given the volume of a cuboid....
Choose two dimensions. Perhaps width and length.
Multiply them together.
Take the total volume and divide by this number.
Now what do you get?

Can you see how to work backwards?

If you can not then simply guess and check to get the required total.