# Subtracting 9 and 19: Teaching Video

Hi parents and teachers! This week I’m working with my kids on subtracting numbers like 9, 19, 8, and 18. We had a couple of “lightbulb” moments with this strategy, so I thought I’d make a little video to share with you. I’m planning on making more of these over the coming weeks, so be sure to be following along on Facebook or Instagram!

The video below will be useful for you or your kids to watch in order to learn this strategy. I also want to make sure that you understand the “whys” behind this strategy, so once you’re finished watching, scroll down to read more about the rationale for this strategy, as well as some ideas that you can use with your kids for taking it even further!

### Who can use this strategy?

Number lines are a useful tool for any age! Personally, I did this lesson with both my 4th grader and my 7th grader, and they both had some lightbulb moments! If your child is comfortable with 2-digit numbers, understands that 9 is one less than 10, and comprehends addition and subtraction, try it!

### Why use a number line?

A number line is an important tool for making math more visual. When numbers are represented on the number line, it makes it easier for kids to “see” them. Once your child works with this strategy for a while, he/she will be ready to move away from the number line and simply do it mentally.

### What if these numbers are too low for my kids?

The great thing about this strategy is that is transfers. The big idea here is flexibility with numbers. If you have an older child, challenge him to see if this strategy would work to solve 128-39, or 276-58. Basically what we are teaching here, is that we can manipulate numbers, make them easier to work with, and then make up for the difference later. I do recommend starting easy though, and then progressing as your child is ready.

### Why not set this up vertically and use “borrowing” like I was taught?

When your child is in older grades, she WILL learn the traditional algorithm using “borrowing.” However, a lot of research has shown us the importance of understanding what the numbers mean before we get to that point. This strategy will help build that essential understanding.

I recommend working with this strategy in-depth for a day or two until your child is showing a solid understanding. Try to extend it to other numbers. Challenge your child to see if it works with other numbers as well. Some ideas for investigating this strategy are:

• “I wonder if this will work with _____________.”
• “I wonder if there are any numbers that this doesn’t work very well with?”
• “Do you think this would work when we subtract a number that ends in 7 or 8? How would that work?”

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