explain why it makes sense for the original amount to be the denominator when you write a proportion to find the percent change

Since while finding a percent change i. e. to find the percent increase or decrease in the quantity the old value or we can say the original value plays a vital role.

" Because Percentage Change is all about comparing old to new values "

If we do not know the original value then there is no means of comparing the new quantity, the new quantity is to be measured with respect to the old quantity. Hence, the original quantity is to be kept in the denominator.

1) First: work out the difference (increase) between the two numbers you are comparing.

2) Increase = New Number - Original Number.

3) Then: divide the increase by the original number and multiply the answer by 100.

4) % increase = Increase : Original Number * 100.