You pay $38.50 for a sweater that is marked 30% off the regular price. what is the regular price of the sweater? how much did you save by buying it on sale?

The regular price is $55.00 and saved $16.50. 

You have to work backwards. To get the sale price you normally have to multiply the original price by the percent (%30 -> .30) then subtract to find the price of an item on sale. 

But, in this case that was done for us, so instead we have to work backwards. 
To do that you have to do $38.50/.70 = $55 (the original price).