Subtraction problem

Consider this subtraction problem

1241
- 587

Most people won't like to do this in their head (or even paper)

This subtraction problem could be simplified by subtracting 600 instead of 587

1241-600 = 641

But, we have subtracted too much, 13 (600-587) too much.
So we add the "missing" 13 to the 641 we have. And we get the answer which is 654.

End of Chapter 0

* If there is a positive feedback, I'll post more of this.

Tips on fast mental calculation

Hey guys, I just bought a book on mental calculation. It is called Think Like A Maths Genius, written by Arthur Benjamin and Michael Shermer. Some of the tricks are damn obvious, but I'll type it here anyway.

Chapter 0
Quick Tricks:
Easy (and impressive) Calculation

Instant Multiplication of 11

71 x 11
To solve this,, add the digits 7 + 1 = 8 in the middle of 7 and 1
71 x 11 = 781

How about
76 x 11
Similar way, 7 + 6 = 13
BUT the answer is not 7136
It should be done this way


1
736+
836

How about 3 digit numbers?
Simple. It can be done in similar way.
Eg. 123 x 11
1 + 2 = 3
2 + 3 = 5
123 x 11 = 1353

Multipying 2 2-digit number with the same first digit and second digit that sums to 10.

71 x 79
The first 2 digit will be 7 x 8 = 56
and The last 2 digit will be 1 x 9 = 9
so the answer is 5609.

*more will be added soon.