This sequence begins with a focus on developing students’ counting skills to work with numbers beyond 10, then continues on to build early place value understanding as students investigate ‘teen’ numbers. The key idea of ‘unitising’ is explored as students use a group of ten and some more ones to represent numbers eleven to nineteen.

This sequence is for students who:

- have developed one-to-one correspondence with numbers up to at least 20.
- have an understanding of the composition of numbers, e.g. 7 can be made by combining 1 and 6.
- have used 10 as a group previously, such as participating in tasks that use models such as tens frames.

This sequence requires students to keep count with static objects, which can be more complex than keeping track of the count using manipulatives.

### Lesson 1: One is a Snail, Ten is a Crab

This task uses the book *One is a Snail, Ten is a Crab* to explore numbers up to 20. Students represent numbers using the animals in the book and look at how one number can be represented in multiple ways. Students also explore the concept of efficiency when comparing various representations.

### Lesson 2: One Crab + Some More

This task continues to use the book *One is a Snail, Ten is a Crab*. It introduces students to the patterns of our place value system and the significance of ten. The key understanding of unitising is introduced by asking students to represent teen numbers using one crab, which is the same as using one ten. In doing so, students move from using 10 ones to 1 ten.