the first side of a triangular piece of wood is twice the length of a second side and the length of the third side is one inch less than the first side if the perimeter is 79 what are the lengths of the sides

One can take the price of the item before the discount is applied and subtract that from  the percentage off and get the cost of the item. 

I find this is the most effective way. 

another method could be if one turned the percentage into a decimal it could be subtracted from the cost of the item.