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OK,
Look at this problem: |
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Start on
the right.
2 - 4 = a problem.
We need to borrow a one
from the next column.
There's a zero in the next column.
Bigger Problem. |
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Hey,
we can't borrow anything from a zero! |
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Now
what do we do? |
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Before
we can borrow a 1 from that zero, |
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it
has to borrow something first. |
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So
it looks to its left. |
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There's
a 4 living there. |
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So
the zero can borrow a 1 from the 4: |
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Now the
numbers on top are:
3 10 2 |
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Now
we have a 10 for the middle number. |
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The
2 on the right can borrow a one: |
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Now the
numbers on top are:
3 9 12 |
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We
can finally subtract that 4! |
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12 - 4 = 8
Write down the 8
and move one column left. |
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9
- 1 = 8
Write down the 8
and move one column left. |
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3
- 0 = 3
Write down the 3.
We're
out of numbers so we're done! |
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WARNING!
- WARNING! - DANGER!
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When
you do a bunch of borrowing like we just did, |
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it
is easy to mix up the columns! |
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So
write BIG and be NEAT! |
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Longer
Distance Example: |
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We
can't subtract the 4 from the 3.
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We
need to borrow 1 to make the 3 a 13. |
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The
problem is, the only place we can get a 1 is from the 2. |
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It's
ALLLLL the way down at the other end of the columns. |
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We
can get it, but it's gonna take a while: |
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Borrow a 1
from the 2
and give it to the zero
to the right of the 2.
That makes the 2 a 1.
It makes the 0 a 10. |
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Now borrow
a 1 from our new 10.
Give it to the zero to the right of the 10.
That makes the 10 a 9.
It makes the next 0 a 10. |
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Now borrow
a 1 from our new 10.
Give it to the zero to the right of the 10.
That makes the 10 a 9.
It makes the next 0 a 10.
(I think I've heard this one before ...) |
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AND FINALLY
...
Borrow a 1 from our newest 10
and give it to the 3 to the right of the 10.
That makes the 10 a 9.
It makes the 3 a 13.
Now we can subtract! |
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13 - 4 =
9.
Write down the 9 and go on. |
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9 - 0 = 9.
Every time.
Here we do that 3 times and go on. |
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1 - 0 = 1.
Write down the1.
We're out of numbers so we're done! |
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copyright 2005 Bruce Kirkpatrick |
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