Lesson 9 Add Three-digit Numbers

    • Let’s practice adding within 1,000.

Warm-up Number Talk: Ten and Some More

Find the value of each expression mentally.

Activity 1 How Did You Add Three-digit Numbers?

Find the value of each expression. Show your thinking.

Activity 2 Analyze and Add

Two students talking.

Noah and Kiran showed how they found the value of .

Noah’s work

Base ten diagram. 5 hundreds labeled 500. 9 tens labeled 90. 15 ones, 10 circled, labeled 10, 5 labeled 5.

Kiran’s work

  1. How is Noah and Kiran’s work the same? How is it different?

  2. Which student found the correct value? Explain or show your thinking.

Practice Problem

Problem 1

Here is how Jada found the value of .

  1. Explain Jada’s error.

  2. Correct Jada’s work and find the value of .