These exercises and activities are for students to use independently of the teacher to practice number properties. Some of these activities would be suitable for homework. Others require follow-up during teaching sessions.

### Prior knowledge

### Background

Many of these activities parallel those in “compatible numbers to 10”. They are designed for students who already have their facts to 10, but need to extend these to 20. However, these students may or may not have had exposure to the algebra aspects of those activities, so they have largely been repeated here. The teaching notes for “compatible numbers to 10” contain information about some significant teaching points embedded in both sets of exercises, so will not be repeated here, but should be accessed and read prior to using these activities.

### Comments on the Exercises

**Exercise 1**

When reviewing this exercise with students it can be useful to ask students “why” some of the problems are false, or do not equal 20. For example, students may come up with reasons like “as both of the numbers are less than 10, then the total must also be less than 20”.

**Exercise 2**

This exercise is in two parts, the second part again reverses the sense of the problems so the sum is on the right of the symbol. This is more challenging and will need to be discussed as part of relevant teaching before setting these problems. In this second set of problems students are doing the reverse of comparing a sum to 20. Rather, they are comparing 20 to the results of their calculation. This reverse sense can be problematic as the students are using inequalities, so may want to use the incorrect sign. Reading the sentence in reverse can alleviate this problem.

**Exercise 3**

**Exercise 4**

**Exercise 5**

**Exercise 6**

**Exercise 7**