You can buy exactly 5 CDs with the gift card.
To find the answer, we first need to find out how much is left on the card. In order to find that, we look at the original amount and subtract the cost of the player.
100 - 45 = 55
Knowing that we have 55 left, we want to divide that by the cost of a CD. This will tell us how many we can buy.
55/11 = 5 CDs