It is interesting, though, that the Wizarding world doesn't place much weight in any of those three categories, but instead spell use, potions ability, and diviniation of some sort.
The easiest way to make change, if you're unable to do the maths, is just to count up from the total until what's in your hand matches the amount you were given. If your total was 3 galleons, 7 sickles and 3 knuts, and you gave him 4 galleons, all he'd have to do is start at 3G, 7S, and 3K and count up from there until he gets to 4 galleons. Though I agree, the maths isn't that difficult and Hogwarts graduates should be expected to do it.