Subtract the ones...  Then, subtract the tens...

Practice some by writing your problems out this way.  Then, you'll really know what's going on.  Once you get really good at it, this is the short-hand way of showing your work (and the way most people do it):

1) Make a scratch through the 7 since you're going to lower it by 1

2) Put the 6 above the 7 (the 6 is what's left when you take 1 ten chunk out) :

3) Put a 1 by the 5, making it 15:

4) Finish it off!

I'm going to use this faster way of showing my work in the next lesson, so you can get used to it!

I've got some practice problems with the faster way on the next page!