First, you need to determine whether the percentages will both be applied to the base amount, or whether they will be applied sequentially. For example, let's say you have an item that costs $20 and you have coupons for 15% off and 30% off. Will the item be discounted by 15% first, with the resulting amount then discounted by 30%? Or will the discounts be applied separately, so that 15% is taken off $20 and 30% is also taken off $20?
This is a bit confusing, so let's look at an example. If the discounts are both taken from the base amount, the math is easy. You just add the two percentages and get the total discount. So if a $20 item is discounted by 15% and 30%, and both those percentages are taken off the original price of $20, the total discount is 45% off $20. 45% of $20 is $9 (20 x 0.45 = 9), which means the item is $9 off. Its price is $11.
Now let's try it with sequential discounts. First, let's say the $20 item is discounted by 15%. 20 x .15 = 3, so that discount is $3. The resulting price, then, is $17. Now let's say the 30% discount is applied to that $17. 17 x .3 = 5.1. That discount, then, is $5.10. $17 - $5.10 = $11.90, so the item's final price is $11.90.