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1993-08-12
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*Most people were taught to
approach an addition problem by
starting with the first column on
the right - the units column.$
In this case, you would add 2, 1,
and 6 to get 9, the units part of
the answer.$
Then you would work with the tens
digits and add 6, 3, and 2,
for a sum of 11.$
You would put the right-hand 1 in
11 as the tens digit of your
answer and carry the left-hand 1
in 11 to the hundreds column of
the problem.$
Then you would add the carried 1 to
the 4, 2, and 3 to get the first
part of the answer - 10. That's one
way to do addition.$
Some people feel it makes more
sense to add in the same direction
we read - from left to right. In
fact, this is so easy, you can add
long columns of numbers in your
head.$
Let's try it out on a simple
problem. Start at the top left, and
keep a running total in your head.$
Add 20, 10, and 30 to get 60.
60 is your base number. It will
change as you move through the
problem. When you get to the end,
the base number will be the answer.$
Next we add the numbers on the right
to the base number. 60 + 5 = 65,
+ 2 = 67, + 1 = 68. 68 is the
new base number, and since we've run
out of numbers to add, it is also
the answer to the problem.$
What if we have to "carry" a number,
as we did in our first example of
right to left addition?$
Start on the left.
40 + 20 = 60
60 + 10 = 70
70 is the base number.$
Go to the next column. Add these
numbers to the base number, and you
have the answer to the problem. Did
you notice the carry?$
With Left to Right Addition, you can
add numbers of any size in your head.
Let's try adding some 3-digit numbers
together. It's easier than you might
think!$
Start on the left.
300 + 100 = 400
400 + 400 = 800
Now let's go to the next column.$
800 + 50 = 850
850 + 20 = 870
870 + 10 = 880
Now move to the last column:$
880 + 2 = 882
882 + 1 = 883
883 + 5 = 888
You just added three 3-digit numbers
in your head!$
But what happens if you have to
carry? Let's see how it works if you
just start adding from the left.$
Starting on the left,
Add: 400 + 100 + 300 = 800$
Go to the next column.
800 + 60 = 860
860 + 30 = 890
890 + 20 = 910
Did you notice the carry?$
Finally,
910 + 2 = 912
912 + 5 = 917
917 + 6 = 923
Pretty easy, huh?@