Eliminate memory-overload and learn 4 math facts on each side of a card! Triangle Flash Cards teach families of facts, showing the relationship between addition and subtraction. How is 4 in one corner relation to 7 and 3 in the other corners? Cover the sum in red, and practice addition. Cover addends in black, and practice subtraction. A total of 20 cards aid in mastery of 160 math facts up to 9+9 and 18-9. For 2 or more players, grades 1+.