Unit 2, Lesson 9

Add and Subtract within 100

Grade 2

Warm-up

Which 3 go together?

Base ten diagram. 3 tens. 10 ones. The tens do not show each unit of one. — A

Base ten diagram. 4 groups of 10 ones. — C

Activity 1

Your teacher will give you a set of cards that show expressions and diagrams.

Match each expression to a diagram. Be ready to explain your reasoning.
Choose 1 addition expression. Find the value of the sum.
Choose 1 subtraction expression. Find value of the difference.

Activity 2

Find the value of each expression. Show your thinking using objects, drawings, numbers, or words.

Student Section Summary

We added two-digit numbers. We composed 1 ten by putting 10 ones together. We also subtracted two-digit numbers. We decomposed 1 ten by breaking it apart into 10 ones. We used base-ten blocks and base-ten diagrams to show our thinking.

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