The total amount of the gift card is the amount left on it plus anything that has been spent so far. So in your problem...
Total original value = 45 + 15 = $60
Another way to look at it would be that you started out with a certain amount of dollars (let's call it x) and subtracted 15 dollars and now you have 45...
x - 15 = 45
x = 45 + 15
x = $60