Multiplying a two-digit number by 11
For instance, suppose you wanted to multiple 43 x 11. Well, you may be able to solve this problem in your head already if you realize that you can first multiply 43 by 10, and then adding 43 to the result. Sure enough, the answer is 473. This works relatively quickly, but there is another method that you can use to get the answer more quickly!
This is a special case for multiplying two-digit numbers by 11. Indeed, the method can be used more generally on larger digit-numbers, but let’s look at how to employ this method with two-digit numbers:
First, take the digits in the number you are multiplying by 11, and add them. In our case (43), it is 4 + 3 = 7. Now take the result, and place in between the two-digit number. In our case, take the “7″ and place it between the “4″ and “3″. Did you see the result? Right! it is the answer 473!
Pretty neat, huh? Keep in mind that other problems may involve carrying-over numbers (like 38 * 11 = 418), but with a little practice, this will become almost natural.
This was a special case for numbers multiplied by 11, but there are many other methods you can use to calculate more complex problems quickly in your head.