Age 11 to 14 Challenge Level:
You're going to have to come up with a system to make sure that you don't miss out any pairs, and to make sure that you don't count any twice. You could pick one card and see how many other cards could be paired with it, to get you started.
Is it possible to pair all the cards? Try playing around with different possibilities. You might have to pair up some cards first, if they don't have many possible partners.