

Easiest method for multiplying numbers - mquaes
http://mathema-tricks.blogspot.com/2011/12/method-for-multiplying-numbers-where.html

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thomc
These kind of tricks are pretty handy, and I don't know why I wasn't taught
them back in school, growing up in the calculator generation. My father could
do all kinds of math in his head, which was just as well since he was a
mathematician, but couldn't explain how he did it, he just "knew" the answer.

I taught myself some tricks after the fact, the rest is just practice I think.
Some examples:

Multiply any two digit number by 11 easily:

Using 62 as an example. Separate the two digits (6__2). Notice the gap between
them Add 6 and 2 together (6+2=8) Put the resulting 8 in the gap to get the
answer: 682, 62x11=682.

If the result of the addition > 9, put the least significant digit in the gap
and carry the most significant digit.

This can be expanded to multiply any number by 11 in your head.

Even really easy/obvious tricks are useful, e.g:

To quickly multiply any number by 5, divide the number in two and then
multiply it by 10. Very quick to do in your head.

