Lesson objective: Extend understanding that we can compose and decompose the numbers 11-19 by starting with ten ones and counting on some further ones.

Students bring prior knowledge that the numbers 11-19 are the sum of ten ones and one, two, three, four, five, six, seven, eight, or nine ones from K.NBT.1. This prior knowledge is extended to counting on as students start with 10 and count on by 1, 2, 3, 4, 5, 6, 7, 8, 9, 10 to form teen numbers. A conceptual challenge students may encounter is understanding that we can count on from a given number rather than counting all.

The concept is developed through work with two ten-strips, which supports the concept of having 10 and counting on by 1, 2, 3, 4, 5, 6, 7, 8, or 9 to create the numbers 11-19.

This work helps students deepen their understanding of operations because students will develop the skill of counting on to find a sum.

Students engage in Mathematical Practice 4-Model with mathematics as they work with double ten strips where 10 are already filled in and counting on by 1, 2, 3, 4, 5, 6, 7, 8, or 9 to compose the numbers 11-19.

**Key vocabulary:**

- add
- addend
- compose
- count on
- decompose
- sum
- "teen" number

**Special materials needed:**

- counters
- double ten frame or double decker bus template
- numeral cards 1-9
- arithmetic rack (optional - these can be used in place of the double decker bus template)