Empty number line for counting on

Counting on using an empty number line is a good method for subtracting numbers mentally.

Use this method to find the difference between 37 and 50. This is the same as the sum
50 - 37.

When you have pictured that line, count on from 37 to 40, which makes 3. Keep that 3 in your head. Then, count from 40 to 50, which is 10.

Have a look below to see how this works.

Now all you need to do is add the 3 to the 10. This makes 13. So:

The difference between 37 and 50 is 13.
Or
50 - 37 = 13