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Anything to speed up or simplify calculations.

A simple example might be to get a multiple of $19$, for instance, $38 \cdot 19 = 38 \cdot 20 - 38$.

(This is hard to tag with so few tags in play!)

mental-calculations tips tricks shortcut cheats time-saver

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To square a number ending in 5:

Remove the ending 5. Let the resulting number be n, and compute n(n+1). Append 25 to the end of n(n+1) and that's your answer.

Example: 852. Here, we drop the last digit to get 8, compute 8*9 = 72, so 852 = 7225. Similarly, we can compute 1152. Here, we drop the last digit to get 11, compute 11*12 = 132, so 1152 = 13225.

How does this work?: Note that (10n + 5)2 = 100n2 + 100n + 25 = 100 * n(n+1) + 25.

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When squaring a number, break the calculation into three smaller calculations and add them using the FOIL method.

Example:

302^2 = 300^2 + 2*(300*2) + 2^2.

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    $\begingroup$ The term "FOIL" should be banned. It makes it seem there is something in it... $\endgroup$
    – Mariano Suárez-Álvarez
    Commented Oct 29, 2010 at 16:26
  • $\begingroup$ @MarianoSuárez-Alvarez I'd hope that as a joke. FOIL seems to consist of a pretty good mnemonic for some people, and also acronyms like this don't have to get pronounced as words, but instead just spelled out, like "F O I L". $\endgroup$
    – Doug Spoonwood
    Commented Jan 15, 2012 at 13:23
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One simple one that you're probably familiar with already: When you multiply a one-digit number, n, by 9, the result has n-1 in the 10s place, and then the ones digit is such that the sum of the digits is 9.

Example: 9*6 = 54 because 5 is 6-1, and then 5+4 is 9.

What's really cool is that you can use this trick, plus your fingers, to get the answer instantly. Hold out your hands with all 10 fingers up, then put down the n'th finger (which might be on either hand. Then the number is just (how many fingers there are to the left of the finger you put down)*10 + (how many fingers there are to the right of the finger you put down)

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Art Benjamin is your man! He has many tricks to speed up mental calculation and other fun mathemagical tricks. He also wrote two books on the subject!

Here is a video of him in action: http://www.youtube.com/watch?v=M4vqr3_ROIk

Here is his new book: http://www.amazon.com/Secrets-Mental-Math-Mathemagicians-Calculation/dp/0307338401

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Multiplying becomes especially easy if the numbers are of the same parity, through $$(a-b)(a+b)=a^2-b^2$$

$$27\times33=(30-3)(30+3)=30^2-3^2=891$$

You can memorize a fairly large number of squares pretty quickly; then you just need to get fast at taking averages (which is why even$\times$odd can be trickier) and subtracting.

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Not sure if you are an iOS user, but...

https://itunes.apple.com/us/app/mathemagics-mental-math-tricks/id306586847?mt=8

That has a plethora of mental math tricks with practice problems...

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  1. Since you mentioned paper... Double entry bookkeeping and other quality control measures... like checking evens/odds on the rightmost digits of a column that was summed, can tell you in a few seconds if a total is wrong. In the Accounting world, quality control is an important component of speed, since over the long term, there will always be mistakes and backtracing them takes more time than if quality checks had been performed during calculation steps.

  2. Touch your thumb to each of the bone segments of your fingers. You can count to twelve with one hand, or to 144 with two hands.

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