For those that love math and how numbers are manipulated using mathematical operations, this article is for you. For those that want to improve their mental math skills, this article is for you as well. Even those that don’t like math can benefit from learning how to do mental math. There are many types of problems one can do mentally, but for the scope of this article, I will concentrate on the simple problem of multiplying any two-digit number by 11. As a math tutor for 14 years, I have taught several students this method in attempts to transition from the use of a calculator for all problems to performing more calculations in the head.
Let’s take a simple example, multiplying 34 by 11. To solve the problem, simply add the digits 3 + 4 = 7 and write the digits between the 3 and the 4. Your answer is 374. Very simply done. You can check your answer by multiplying by hand or using a calculator. Let’s take another example, 42 times 11. Add the 4 +2 = 6 and write 6 between the 4 and 2. The answer is 462. Now that was easy, but don’t get overly excited just yet. There are other cases where a little more needs to be done.
For example, suppose the problem is 67 times 11. Although 6 + 7 = 13, the answer is not 6,137. As before the 3 goes in between the numbers, but the 1 needs to be added to the 6 to get 7. The correct answer is 737. Basically think of adding the 1 to the 6 as “carrying” the 1 like you would in a simple problem such as 13 + 8, add the 3 and 8 to get 11, write down the 1 and carry the 1, adding it to the other 1 to get 21. Let’s try another example, suppose the problem is 84 times 11. Add 8 and 4 to get 12. Write the 2 in between the 8 and 4 but remember to carry the 1, adding to the 8 to get 924. See how easy that is?
This is just one of several articles I plan to write on mental math. Others will include mental addition and subtraction, basic multiplication, mental division, estimations and advanced multiplication. In the meantime, practice the mental math you learned in this article. It will exercise your mind and impress your friends!