Another way to calculate two percentage discounts?

If I want to calculate two discounts, say I get 15% off of an item from a sale but I also get 20% from a coupon. Correct me if I'm wrong but I wouldn't add 15 and 20 together; I would subtract the 15 from 100 percent to get 85% (or .85) and then multiply that with 80% (.80) I get from subtracting 20 from 100 to get product which is .68 (.85 x .8). Then you subtract that from 1 to get .32 and that is the total discount. That works, but it's a tad tedious. Is there another way to get 32%, preferably just using 15 and 20?