As adults, we see the relationship between addition and subtraction, and between multiplication and division - but these relationships are not so evident to the young mathematics student. One of my favorite ways to demonstrate this relationship is by playing a very fun game called Salute!
Divide the deck between two of the players. The third player says, Salute! and the other two bring the first card in the piles to their foreheads, without looking at it, in a kind of salute.
The two players look at one another's foreheads as player 3 adds the numbers. He then says, "The sum is ....." The two players need to figure out their own number by subtracting the other player's number from the sum.