secrets-of-mental-math-malestrom

Consider the subtraction problem
Quick Tricks: Easy (and Impressive) Calculations 7
1241
587
Most people would not like to do this problem in their head
(or even on paper!), but let’s simplify it. Instead of subtracting
587, subtract 600. Since 1200 600 600, we have that
1241
600
641
But we have subtracted 13 too much. (We will explain how to
quickly determine the 13 in Chapter 1.) Thus, our painfullooking
subtraction problem becomes the easy addition problem
641
13
654
which is not too hard to calculate in your head (especially from
left to right). Thus, 1241 587 654.
Using a little bit of mathematical magic, described in Chapter
9, you will be able to instantly compute the sum of the ten numbers
below.
9
5
14
19
33
52
85
137
222
359
935