Don’t you just hate trying to subtract 99 from a number like 4000?
And teaching it is even more difficult. You find you have to regroup several times. Here is an easy way that I never knew about growing up. Say you have a problem like the ones below. Here’s the secret. If you rewrite the problem and lower the subtrahend (the top number) and minuend (the bottom number) by one, you get the same answer and it makes subtracting much easier.
Look at these examples and you’ll get the idea!
Isn’t that easier than regrouping the numbers across all the zeros? It’s good practice for children to count backwards too. Of course children need to learn regrouping but this is what I’d call short-cut subtraction.
It’s just a fun time-saver when there are a lot of zeroes. Students love to explore why it always works. Use a group of 20 beans. Take away 17 and the answer is 3. Set up a group of 19 beans and take away 16. What’s the answer? Again, it’s 3.
Students have a much greater understanding when they can visualize math problems.
I’ve been sharing this fast method while speaking at teacher conferences, and I’ve been amazed at how many teachers don’t know about it. Why do something the hard way when there is a simpler method?