secrets-of-mental-math-malestrom

172
Secrets of Mental Math
Then do the 4-by-1 problem of 3,901 9 for your final answer
of 35,109.
The Addition Method
If the two-digit number and the three-digit number cannot be
conveniently factored in the 3-by-2 problem you’re doing, you
can always resort to the addition method:
721
37
720 37 26,640
1 37 37
26,677
(720 1)
(treating 72 as 9 8)
The method requires you to add a 2-by-2 (times 10) to a
2-by-1. These problems tend to be more difficult than problems
that can be factored since you have to perform a 2-by-1
while holding on to a five-digit number, and then add the results
together. In fact, it is probably easier to solve this problem by
factoring 721 into 103 7, then computing 37 103 7
3,811 7 26,677.
Here’s another example:
732
57
730 57 41,610
2 57 114
41,724
(730 2)
(treating 73 as 70 3)
Though you will usually break up the three-digit number when
using the addition method, it occasionally pays to break up the