secrets-of-mental-math-malestrom

170
Secrets of Mental Math
numbers 11 and below, so you’ll be able to use this method for
many problems. Here’s an example:
853
44
(11 4)
853 11 4 9,383 4 37,532
To multiply 853 11, you treat 853 as 850 3 and proceed
as follows:
850 11 9,350
3 11 33
9,383
Now multiply 9,383 4 by treating 9,383 as 9,300 83, as
follows:
9,300 4 37,200
83 4 332
37,532
If the two-digit number is not factorable into small numbers,
examine the three-digit number to see if it can be factored:
144
76
(6 6 4)
76 144 76 6 6 4 456 6 4
2,736 4 10,944
Notice that the sequence of the multiplication problems is a
2-by-1, a 3-by-1, and finally a 4-by-1. Since these are all prob-