If item costs $99.99 with 15% off and I get final price. It then asks me what percentage of savings. How do I get the percentage of savings

ok.. if you got 84.99 as you answer then you would put .15 *99.99 then you would get 14.9985 then round and get 15.00 and then 99.99 - 15.00 ....you will get 84.99...i think