Discuss with students that there are different addition problems whose total is 20, and show this on the interactive whiteboard with the child, grandpa and 20 candies. The candies are divided differently every time. Show all of the divisions and discuss the addition problem that is represented by each division. There are only 20 candies that are being split differently each time, so the total of all the different divisions is still 20.
Next, show the pencils. Say that you can add the pencils by counting the pencils in the first group, and then counting on with the pencils in the second group. Count the pencils aloud. Do not count the second group as a separate group, but count on from 18. Practice with two more examples with sandwiches and soda to add to 20 by counting on.
Explain that there are also addition problems in which you do not have objects to count, but that you use numbers to represent the objects. You still count the second number with the first. Students can use blocks to count, or blocks to represent the first addend for support. Discuss two addition problems as a class whose total is 20 without visual support.
Next show the pencils to the students again. This time, they aren't asked to add the pencils, but to discover how many more they need to fill a box of 20 pencils. Explain that you can calculate how many more you need to count on to 20 from the first addend. Practice this again with lollipops and candies and have them count on to reach 20 and then write down that number as the second addend. You can also do this with addition problems in which you can't see count the objects, you just get the first addend and count on to reach 20. Discuss the exercises given as a class.
Check that students understand addition to 20 by asking the following questions:
- How do you add two addends together?
- What do you do if you want to discover the missing addend in the problem: 18 + ... = 20?