ffirs.indd

204 Appendix A
If you were asked to subtract 378 from 613, you would again write
the problem down as usual. Th en you could fi nd the answer in your
head by subtracting 400 and adding 22.
613 – 400 = 213
213 + 22 = 235
Using this method there is no carrying or borrowing. You just write
the answer and then check by casting out nines.
Th e same goes for all mental calculation; that is, all calculations
where you write nothing down. No one knows what method you
use. Students and teachers have always used diff erent methods, even
for such simple problems as 56 minus 9, as I pointed out earlier in
the book.
Also, the higher the grade you are in, the less of a problem this
will be.
Now let’s look at where it can make a diff erence. Let’s say you are
asked to multiply 355 by 52. You set out the problem as the teacher
requires.
355
× 52
710
17,750
18,460
Here is the fi nished calculation. Can you now make use of what you
have learned in this book? Certainly you can. You can cast out nines
to check your answer.
355 × 52 = 18,460
13 7 10
4 1
Th e substitutes are 4 × 7 = 1, which we can see is correct because 4
times 7 equals 28, and 2 plus 8 is 10, which adds to 1.
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