Multiplying by 11

[gamesandstuff]

It's very easy to multiply by 11. A more useful application of this is checking divisibility with 11.
Starting from the leftmost digit, subtract/add each following digit in a repeating pattern.
If the result is divisible by 11, the original number is too.

[Mycreativeworld..]

How about 4 dugit numbers like 6578×11

[alexp]

Or you can simply 3240+324 3564 which takes like a second to calculate in your head