20% in decimal form (and the form we need to do math with) is 20/100 or 0.20 (because there’s 20 units for every cent; 20 for every hundred).
You multiply 15 (the total cost) by 0.20 to get 20% of 15, then add 15 to 20% of 15 to get the total cost.
0.2(15) + 15 = 18
This works because you are multiplying 15 by one fifth of its value (20%) then adding the actual cost to it.
