secrets-of-mental-math-malestrom

38
Secrets of Mental Math
sometimes especially easy to solve. The same is true for 3-by-1
problems:
563 (500 60 3)
6
500 6 3000
60 6 360
3 6 1 8
3378
Notice that whenever the first product is a multiple of 1000,
the resulting addition problem is no problem at all. This is
because you do not have to carry any numbers and the thousands
digit does not change. If you were solving the problem
above in front of someone else, you would be able to say your
first product—“three thousand . . .”—out loud with complete
confidence that a carried number would not change it to 4000.
(As an added bonus, by quickly saying the first digit, it gives the
illusion that you computed the entire answer immediately!)
Even if you are practicing alone, saying your first product out
loud frees up some memory space while you work on the
remaining 2-by-1 problem, which you can say out loud as
well—in this case, “. . . three hundred seventy-eight.”
Try the same approach in solving the next problem, where
the multiplier is a 5:
663 (600 60 3)
5
600 5 3000
60 5 300
3 5
1
5
3315