I explained here about how to multiply any two digit number by 11 instantly.
Just to recap, you take the two numbers, add them together and then put that number between the original pair of numbers.
For example, 72 x 11
7+2 = 9
Put the 9 between 7 and 2 and the answer is: 792
72 x 11 = 792
But what do you do, when the numbers add up to a digit more than 10?
For example, 85 x 11?
8 + 5 = 13, but the answer to 85 x 11 is NOT 8135
The answer is you take your 13, and put the five in the middle as normal, but add the one to the eight.
85 x 11 = 935
Think of like this, with the 13 (i.e. the result of adding together your two numbers) in red.
1
835
935
Another example: 75 x 11
7 + 5 = 12. … Put the 2 in the middle and add the 1 to the first digit. …
75 x 11 = 825
Voila!