Multiplication By Eleven

Two Digit Numbers

This trick is quite simple! All you have to do is add the tens and ones digit, then stick that sum in the middle of the two numbers.

Example 1: Solve 35 * 11.

Step 1: 3 + 5 = 8
Step 2: Now, we just add the '8' to 35 to calculate, 385. Simple!

Example 2: Solve 59 * 11.

Step 1: 5 + 9 = 14
Step 2 (Bogus Solution): The answer is 5149.
Incorrect! If your sum is larger than or equal to 10, you must "carry over" the tens digit. Step 2 (Correct): In this case, the tens digit (14) is 1. So, instead of sandwiching "14" between 5 and 9, we carry the one to the 5 to obtain 5 + 1 = 6. However, regardless of this change, the ones digit remains after the ones digit (9) but before the tens digit, which is now 6.
Our final answer is: 649.

Three Digits (or more!)

With relatively the same concept, you can level up to multiplying three digit numbers (or more!) by 11. First, leave the first and last digits and add up the rest together, in this fashion: