Find the difference between 40 and 20. Use the dot cards to help you.
The difference from 40 to 20 would be like starting with 40 and wanting to end with only 20. Right now on the screen, there are 40 red dots from our four dot cards. How many dots would we have to take away to end up with 20? Remember, we’re starting with 40. Let’s see what would happen if we took away 10.
Now we’ve taken away 10, and we only have 30 dots remaining. We’re trying to figure out how many we need to take away to get to 20. Let’s take away 10 more. Now we’ve taken away an additional 10, and we have 20 red dots remaining. We took two dot cards away. 10 plus 10 equals 20. We started with 40 and took 20 away, and that tells us that the difference between 40 and 20 is 20.