Consider the following simple multiplication:

2 x 3 = 6

Now consider that the first term is increased by 50% to 3 and the second term is decreased by 67% to 1:

3 x 1 = 3

The total has decreased by 3, or 50%. To what extent does the increase of 2 to 3 account for this change and to what extent does the decrease from 3 to 1 account for the change?