secrets-of-mental-math-malestrom

New and Improved Products: Intermediate Multiplication 69
APPROACHING MULTIPLICATION CREATIVELY
I mentioned at the beginning of the chapter that multiplication
problems are fun because they can be solved any number of
ways. Now that you know what I mean, let’s apply all three
methods explained in this chapter to a single problem, 73 49.
We’ll begin by using the addition method:
73
49
70 49 3430
3 49 147
3577
Now try the subtraction method:
73
49
50 73 3650
1 73 73
3577
(70 3)
(50 1)
Note that the last two digits of the subtraction could be
obtained by adding 50 (complement of 73) 50 27 77
or by simply taking the complement of (73 50) complement
of 23 77.
Finally, try the factoring method:
73 49 73 7 7 511 7 3577
Congratulations! You have mastered 2-by-2 multiplication
and now have all the basic skills you need to be a fast mental
calculator. All you need to become a lightning calculator is more
practice!