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compmul.txt
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1993-08-13
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*Sometimes, you can take a shortcut.
Let's look at a few examples of one
such shortcut: complementary multi-
plication.$
Let's try 96 x 94. Both these numbers
are close to 100, so here's how we do
it. How far is 96 from 100? Put that
number next to 96. How far is 94 from
100? Put that number to the right
of 94.$
The next step is to subtract dia-
gonally. Subtract 4 from 94 or 6
from 96. In either case, you'll get
90. 90 is your base number.$
Now multiply the two numbers you put
on the right: 6 x 4 = 24
Put this number to the right of the
90, and you have the answer:
96 x 94 = 9,024
Easy!$
This method also works well for
numbers close to 50, though there
is an extra step. After either
adding or subtracting diagonally,
divide the result by 2. Then, do
as before.$
The problem is 48 x 44.
How far is 48 from 50? 50 - 48 = 2
How far is 44 from 50? 50 - 44 = 6
Put these numbers to the right of
the problem.$
Now, subtract diagonally in either
direction, 48 - 6 or 44 - 2.
Either way, you'll get 42.$
Since you are multiplying numbers
near 50 (1/2 of 100), divide 42
by 2, giving 21. Put this under
the problem as the first part of
the answer.$
Then, multiply the two numbers you
put on the right, and put them down
as the second part of the answer.
Simple! 48 x 44 = 2112$
For numbers near multiples of 100,
after adding or subtracting dia-
gonally, multiply that result by
the appropriate multiple. Let's
try 203 x 206.$
Find how far each number is from 200
and put that number to the right.$
Since these numbers are greater than
200, we'll cross-add:
203 + 6 or 206 + 3
Either way, we get 209.$
Multiply 209 by 2 (200 is 2 x 100)
and put it down as the first part
of the answer.$
Finally, multiply the two numbers you
put on the right (6 x 3 = 18), put
that result down as the second part
of your answer and you're done!
203 x 206 = 41,818@