Intro: How to Multiply by 11 Quickly
Multiplying by numbers less than 11 is easy. However, using the fact that 11 = 10 + 1, there is a way to multiply by 11 quickly, in less than 15 seconds.
Step 1: Prepare to Add
Put down the last digit of the number on the right-most side of the answer, like the image shows.
Step 2: Add the Side-by-side Digits
Add the second last digit of the number you want to multiply with the last digit.
If the number you have is more than 10, do not add the carry yet. The carrying will start in the next step.
The image has a carry.
Step 3: Continue Till the First Digit
Do the same thing on step 2 and add the third last digit and the second last digit together, and so on, until you reach the first digit. Add the carry if it is needed. The carry is for numbers more than 9 only.
Now, the multiplication by 11 has been finished and the result is correct.