Lesson 5

In grade 1, students added within 100 using strategies based on place value and properties of operations. When using place value strategies to add, students learned that sometimes they need to compose a ten. To this point, when subtracting within 20, students used methods like counting on, decomposing a number to get to a ten, and using known addition facts, but did not explicitly decompose a ten.

In this lesson, students subtract a one-digit number from a two-digit number when a ten would need to be decomposed if they subtract by place. When subtracting a one-digit number from a two-digit number, students may count back to get to a ten and then count back from there. Although subtracting or counting back in this way is an effective method when subtracting a single-digit number, it is less practical when subtracting two-digit numbers. For this reason, students are encouraged to use cubes and base-ten blocks to help make decomposing a ten visible to all students and prepare students to consider decomposing the minuend in order to subtract by place. For example, \(26 = 10 + 16\), so decomposing 26 into 1 ten and 16 ones will be helpful when representing subtraction using base-ten blocks later in this lesson.

Activity 2: Subtract with Base-ten Blocks