It certainly looks like the "Rule of Three". Bruce has given you the way to solve it.

In a proportion such as

16   12
-- = --
28    X

the cross-product is the same in both cases.

For example, in

1   2
- = -
2   4

1 * 4 = 2 * 2

It works the same for an unknown; in the case of the first proportion above: 16 * X = 12 * 28.

Of course, you need not explain advanced algebra to a child in primary school; just tell him to cross-multiply, and then divide by the number diagonally opposed to the "X".

>Yeah, it's hard to explain when I cant draw the boxes very well. Let me try again.
>
>

>An example:
>
>You can use more than one "work" column to get to the result.
>
>. Known   Work    Work    Result
>---------------------------------
>|  16   |  48   |  24   |  12   |
>|--------------------------------
>|  28   |  84   |  42   |   X   |
>---------------------------------
>
>First row logic:
>
>(((16 * 3) / 2) / 2) = 12 (there are other solutions other than * 3 / 2 / 2)
>((48 / 2) / 2) = 12
>24 / 2 = 12
>
>
>Use same logic on second row:
>
>(((28 * 3) / 2) / 2 = X
>((84 / 2) / 2) = X
>42 / 2 = X
>21 = X
>
>
>The one I can't solve without fractions or decimals.
>
>This is how the table looks (could be more than one "work" column though).
>
>. Known   Work    Result
>-------------------------
>|  50   |       |  18   |
>|------------------------
>|  60   |       |   X   |
>-------------------------
>