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div.txt
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1993-12-10
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*If subtraction is reverse addition,
then division is "reverse multipli-
cation." We want to find out how
many times we have to multiply one
number to get another number.
Let's look at 486 ÷ 18.$
It's easy to simplify some problems
based on the fact that dividing a
number by the parts of a divisor,
called "factors," gives the same
answer as dividing it by the whole
divisor.$
Since 18 is the same as 6 x 3,
we can use these factors to our
advantage. It's easier to divide
486 by 6 and then divide the re-
sult by 3 than it is to divide 486
by 18. 486 ÷ 6 = 81.$
Now, divide the result by 3.
81 ÷ 3 = 27
486 ÷ 18 = 27$
When your divisor ends with one or
more zeros, it's easy to simplify
the problem. Canceling zeros is the
same as dividing both the divisor
and the dividend by tens.$
Drop the zero or zeros from the
end of the divisor, and move the
decimal point in the dividend one
place to the left for each zero
you dropped.$
Since there was no decimal point in
the dividend, we pretended it was at
the very end of the number. Now you
can divide 24.6 by 2. The answer is
12.3 ("twelve and three-tenths").$
Canceling zeros works because mul-
tiplying or dividing both terms by
the same number doesn't change the
answer to the division problem! We
can do this trick using other
numbers besides ten.$
Given the problem, 242 ÷ 5 = , we
can multiply both terms by 2, so
that the problem becomes:
484 ÷ 10 = $
Now cancel the zero to complete the
problem. Move the decimal point at
the end of 484 one place to the
left.
484 ÷ 10 = 48.4
242 ÷ 5 = 48.4$
One of the big headaches in di-
vision is figuring out whether one
number can be divided by another
without having a remainder in
the quotient. Here are some tricks
for the trickier numbers.$
Add up a number's digits. If the
sum has more than one digit, you
can keep adding the digits until
you have only one. If that digit is
a 3, then the original number can
be evenly divided by 3.$
To check a large number for divis-
ibility by four, just look at the
last two digits. If they're both
zeros, or if they form a number
that's divisible by four, the whole
number is evenly divisible by four.$
Add up the digits the same way you
did to see if your number was di-
visible by three. If the digits add
up to nine, your number is evenly
divisible by nine.@