Adding Shortcut

Date: 05/05/2003 at 22:29:25
From: Jessica
Subject: Finding a sum

Explain how 10 helps you find the sum of 9 and 8.

Date: 05/06/2003 at 17:06:22
From: Doctor Peterson
Subject: Re: Finding a sum

Hi, Jessica.

One way to add a pair of numbers like this is to go to ten and then 
see how much more is left to add. I'll demonstrate by adding 8 + 5:

We start with 8. That is 2 less than 10, so if we add 2 to the 8, we 
get to 10. But we want to add 5; that is 3 more than 2. So after we 
add 2, we have 3 more to add to the 10; and the answer is 13.

Here is a picture:
                                      ________5________
                                     /                 \
                    8                   2         3
    +-------------------------------+------>|---------->|
    +---+---+---+---+---+---+---+---+---+---+---+---+---+---+
    0   1   2   3   4   5   6   7   8   9  10  11  12  13  14

It's a lot easier to think about than to write!

If you have any further questions, feel free to write back.

- Doctor Peterson, The Math Forum
  http://mathforum.org/dr.math/