Reddit reviews Speed Mathematics: Secret Skills for Quick Calculation
We found 2 Reddit comments about Speed Mathematics: Secret Skills for Quick Calculation. Here are the top ones, ranked by their Reddit score.
Just change your reference number to something that can be used easily, such as 50, 20, 10, 100, 1000, etc. Then you just take the numbers, such as 45 and 55, for ease of use, set 50 as the reference. 45 is 5 below 50, so -5 for its # and +5 for 55. Then add their number to the opposite number, so +5 to 45 or -5 to 55; it always comes out the same. You get 50; multiply the result with your reference number, so that gets you 2500. Then to tie it all off, add the product of your 45 and 55's initial differences from the previous result, +5 (-5) = -25. 2500 + (-25) = 2475.
It's easier to see if you write it out like that guy has it
50) 55 45
50) 55 45
+5 -5
50) 55 45 (The product of [the sum of one of the differences and the
+5 -5 opposite multiplicand] and the reference number)
50) 55 45 2500
+5 -5
50) 55 45 | 2500
+5 -5 |+ (-25)
-------
2475
Like we've said in this thread, it's not magic. It just takes advantage of the distributive property, given you have a good grasp of decimal (i.e. base 10) mathematics.
Speed Mathematics by Bill Handley
To anyone interested, this method is used in this book, as well as other mathematical shortcuts.
Enjoy.
http://www.amazon.com/gp/aw/d/0471467316