Composing and decomposing a quantity is important in students' development of number sense. Composing is combining the "parts" to make the whole while decomposing a number is "breaking the whole into parts." This part-part-whole conceptualization of numbers develops flexibility in working with numbers and it helps students develop an understanding of the relationship between addition and subtraction as well as place value.
 

Opportunities to compose and decompose quantities below 10 are crucial in order for children to understand relationships among larger numbers. If students understand the relationship between 5 and 8 (8 is 5 and 3 more), they will use this knowledge and make the connection that 18 is 15 and 3 more.
 

Suggested Experiences:

• Students benefit from composing and decomposing teen numbers using snap cubes. Give students a target number and ask them to show different combinations that make the whole. (e.g., several combinations for 16 include 8 and 8; 9 and 7; 10 and 6).
• On the overhead projector, represent a number on a transparency of a double ten-frame and cover part of the quantity. Ask students to look at how many counters are visible, then tell how many counters are covered, and how they figured it out.
• Ask students to decompose teen numbers using 10 as a benchmark. For example, the number 14 can be visually seen by students as 10 and 4 more on a double ten-frame.