One of the things that is important (I think!) in being strong at what we call Mental Math, is being able to see, in your head, the "partner numbers" that add up to, say 10. 10 is particularly important, being the base for the number system we use.
This card game is based on the old standard, Memory. In the original game, a deck of cards is dealt face down on the table, and players take turns trying to turn over matching pairs. It might be numbers, or pictures, or some other representation on the cards.
In this game, we use a standard deck of cards, but only the numbers from 1 (Ace) to 9, so 36 of the 52 cards. Players try to turn over two cards whose values add up to 10. It's a great way to become comfortable with knowing that, for instance, "if I have a 3, I need 7 to get to 10".
Once working with 10 as your total becomes too easy, try taking out the nines, and turning over pairs that add to 9. Do the same for 8 or 7, by taking out all cards of a value equal to or higher than the total you are working with. Note that the number of cards on the table becomes a bit small for a good game as total gets lower.
I have created an instruction sheet for Memory Plus if you need it.
Enjoy!