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Math Facts Games:

Magic Multiplication


Today's Snack: One of the most magic foods is pasta. It is tremendously different after it is cooked, than before. Today, make yourself a serving of macaroni and cheese, but measure the dry macaroni before you cook it, and then measure the finished produce . . . right before it magically disappears, because you ate it! Enjoy a glass of milk with your treat.





Scratch paper and pencil


Here are some secret shortcuts to help you multiply large numbers in your head very quickly - almost as if by magic!


Get into pairs or small groups and study these together. Then send one member of your team up to the board against a member of the other team. Have the adult give you each a question - either some number multiplied by 11, or squaring a number that ends in 5. Whoever writes the answer on the board first gets a point. Play to five points, and you want, go another round and compete for the best two out of three.


Try to do these without paper and pencil so that the habits can be formed without them. But if it's too hard, jot down the steps on paper as you go.


1.      Multiplying a two-digit number by 11.


Let's say you want to multiply 26 x 11. First, separate the two digits of the 26 into 2 and 6. Then, add those two digits together: 2 + 6 = 8. Then put that sum, 8, in between the two digits of 2 and 6. The answer is 286. Check it, and see!


Note that if the sum of the two digits is higher than 9, you will have to carry the "1" as if it is a 10, and add it to the first digit. For example, 84 x 11. First, separate the 8 and the 4. Next, add them together: 8 + 4 = 12. That's more than 9, so keep the 2 in place, and carry the "1" as if it is a ten, and add it to the 8. The answer is 924. Check it, and see!



2.      Squaring a two-digit number ending in 5.


To square a number, you simply multiply it by itself. For example, to square 75, you multiply it this way: 75 x 75. To get the answer really fast, do this: multiply the first digit (7) by one more than itself (8). 7 x 8 = 56. Put a 25 after that - 5 x 5 is always 25, so any two-digit number that you multiply by itself will always end in 25. The answer: 5,625.




By Susan Darst Williams Math 2010

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