“A good decision is based on knowledge and not on numbers.” – Plato
Mental arithmetic has quickly become a little appreciated skill. We can always just “google” the answer – that is as long as we’re within “google-range” or “google-shot” as the case may be. In any case, here’s a really neat and easily mastered mental arithmetic skill when multiplying by the number 11.
First let’s look at the traditional way of multiplying by 11, let’s say 23 x 11.
So, 23 x 11 = 253. I’m confident that we call could have done that with pencil and paper. Also, with a little patience I’m sure many of us could have performed this mentally. However, here’s the “trick” when multiplying by 11.
The product (23 x 11) starts with “2”, ends with “3” (just like “23”) and has “2+3″ or “5” in the middle. Prettly simple, right? Well let’s try another, 45 x 11. That will start with “4”, end with “5” (just like “45”) and have “4+5″ or “9” in the middle. Therefore, 45 x 11 = 495. Here’s a few more for you to practice with:
- 41 x 11 =
- 63 x 11 =
- 18 x 11 =
Using the method just illustrated, the answers are 451, 693 and 198 respectively. But, what happens if the two digits sum to a number 10 or larger? Here’s an example, 57 x 11.
In this example, the two digits (5 and 7) sum to 12, a 2-digit number. In this case, just “carry” the “1” and add it to the “5”. So the answer starts with “5+1″ or “6”, ends with “7” and has “2” in the middle. Realizing this, multiplying 69 by 11 can easily be computed mentally. The answer will start with “6+1″ or “7” end with “9” and has “5” in the middle, which equals 759. Here are a few for you to try:
- 48 x 11
- 74 x 11
- 38 x 11
In each of these problems, “1” must be carried to the first digit of the product because the sum of the two digits is greater than 9. The answers are 528, 814 and 418 respectively.
Practice multiplying by 11 mentally and then try it on others. You’ll quickly discover that you can multiply by 11 faster than a calculator! Good luck and have fun!
