User:Rayhan/Math shortcuts

Suppose you want to quickly multiply any number with 11, there is an easy way:

352х11

Leave spaces between the first and the last digit:

   3 (5) 2
   3+5, 5+2

let the first digit stay where it is. Put the sum of the first digit and the middle digit after the first digit:

   3(8)   5+2 

do the same thing with the middle and last digit:

   3(8)   (7)2

The answer is 3872.

Another example: 2645х11

2[  2+6 6+4 4+5   ]5
=    8  10   9

we cannot put 10 directly, so you have to carry the "1" of 10 should be added to 8.

2[  8+1 ← 0  9    ]5
2[    9   0  9    ]5
 =29095

Ok, this method is more useful for multiplications of smaller units such as the first example, as it saves time.

Contributed by Rayhan