To multiply any two-digit number by 11, simply add the digits of the number together and then put this sum between the original two digits. For example, to quickly find the answer to 11 x 53, start by adding the two digits of the number 53 together to get 5+3=8. Next, put this new number between the original two digits to get 583. That’s the answer!
If the sum of the two digits is greater than 9, carry the 1 over to the tens digit. So for a problem like 11 x 94, start by adding the two digits of the number 94 together to get 9+4=13. Since this number is greater than 10, you need to stick the 3 between the two original numbers and carry the 1 from 13 to the resulting tens column. Doing so gives you an answer of 1,043.
For more, see 2 Tips for Multiplying by 11