Basic Subtraction

Date: 06/08/2001 at 19:09:04
From: nicholas
Subject: Basic Subtraction

I need help with basic subtraction, like 15 - 9 =_____ ?

Date: 06/08/2001 at 22:35:25
From: Doctor Peterson
Subject: Re: Basic Subtraction

Hi, Nicholas.

The main thing you need is just a lot of practice, until you've 
memorized the whole addition table. If you know that 9 + 6 = 15, you 
can remember that 15 - 9 = 6, because that's just another way to say 
the same thing. If you don't know that fact, then you have to work a 
lot harder.

There are ways to work these things out, though, until you've learned 
it all well enough. For example, you can picture this problem on a 
number line:

          9  10  11  12  13  14  15
    ... --+---+---+---+---+---+---+-- ...
          +-->+------------------>|
            1          5

I know that 9 is less than 10; and in particular, I know that it is 1 
less than 10, because I've made sure to learn all the "_ + _ = 10" 
facts. I also know that 15 is 5 more than 10, because that's what 15 
means. So to get from 9 to 15, I can first take 1 step to 10, and then 
5 more to 15, making a total of 6.

That's a lot of thinking for one subtraction, so it won't be too fast. 
But knowing you can work out the answer when you're not sure can make 
you a lot more confident about math! It also gives you a chance to get 
to know numbers better, by playing little games like this with them, 
and that's good for your understanding of math.

You might find it interesting to take a set of subtraction flash 
cards, and put in one pile the facts that are easy (like 15 - 10), and 
in another the ones you have to memorize. You'll be surprised how many 
easy ones there are. You can also look for ways to use the easy ones 
to do the hard ones, as I just did. Then you'll probably be left with 
a few you'll just have to memorize; do that and you're on your way.

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