# Lesson 18

Money Problems

## Warm-up: How Many Do You See: Groups of Coins (10 minutes)

### Narrative

The purpose of this How Many Do You See is for students to use subitizing or grouping strategies to describe the images they see. This is also an opportunity for students to use the names of coins to describe the images. They have the opportunity to name the coins without thinking about the value of the set, making a distinction between the number of coins and their values. As an extension of this discussion during the synthesis, consider asking the value of each set of coins. Each group has the same number of coins, but a different value.

### Launch

• Groups of 2
• “How many coins do you see? How do you see them?”
• Flash the image.
• 30 seconds: quiet think time

### Activity

• Display the image.
• 1 minute: partner discussion
• Record responses.
• Repeat for each image.

### Student Facing

How many coins do you see? How do you see them?

### Activity Synthesis

• “What helped you figure out the number of coins quickly?”
• “All of these images had a total of 8 coins. Does that mean they all have the same value? Explain.” (Each of the coins are worth different amounts, so the values are different. 8¢, 40¢, and 80¢)

## Activity 1: Shop for School Supplies (15 minutes)

### Narrative

The purpose of this activity is for students to solve Add To and Take From problems in the context of money. Students determine how much money each person has and how much money they will have left after buying school supplies. The choice of coins in each problem invites students to consider both concrete and abstract methods for finding the amount of money left after each purchase. For example, students may consider which coins they could remove from the group of coins to purchase the items in each problem or they may just subtract the price of the item from the total value of the coins (MP2). In the synthesis, students discuss different methods they used for finding how much money was left.

MLR5 Co-Craft Questions. Keep books or devices closed. Display only the price list table, without revealing the questions, and ask students what possible mathematical questions could be asked about the situation. Invite students to compare their questions before revealing the task. Ask: “What do these questions have in common? How are they different?” Reveal the intended questions for this task and invite additional connections.
Engagement: Provide Access by Recruiting Interest. Provide choice and autonomy. Provide access to plastic or actual coins to represent the money, and base-ten blocks to add and subtract with a concrete manipulative if needed.
Supports accessibility for: Conceptual Processing, Organization

### Launch

• Groups of 2
• Display the price list table.
• “What do you notice? What do you wonder?” (Students may notice: pencils are the most expensive item on the list. Pens cost a lost less than pencils. Students may wonder: How many pencils come in a pack? What type of eraser is it?)
• 1 minute: quiet think time
• Share and record responses.

### Activity

• “Lin and Diego went to the school store to get a few supplies. Solve each problem on your own, and then compare with your partner.”
• 5 minutes: independent work time
• 5 minutes: partner discussion
• Monitor for students who cross out the coins needed to find how much was left.
• Monitor for students who write equations.

### Student Facing

items cost
pack of pencils 75¢
pencil sharpener 35¢
eraser 45¢
pens 18¢
1. Lin has these coins:

1. How much money does Lin have for supplies?

2. If Lin buys an eraser, how much money will she have left? Show your thinking.

2. Diego has these coins:

1. How much money does Diego have for supplies?
2. If Diego buys a pack of pencils, how much money will he have left? Show your thinking.

### Student Response

If students find total values that do not match the value of the set of coins or have a different amount left after purchases, consider asking:
• “Can you explain your representation to me?”
• “How did you find out how much money (Lin or Diego) had left after purchasing (an eraser or pack of pencils)?”

### Activity Synthesis

• Invite previously identified students to share how they crossed out coins to see how much was left.
• “What did _____ do to solve the problem?”
• Invite previously identified students to share equations they wrote to solve the problem.
• “What did _____ do to represent their thinking?”
• “When did you use addition in the problems? When did you subtract? Why?” (When I was finding how much money, I added. When they bought something, I had to subtract.)

## Activity 2: Shop with a Dollar (20 minutes)

### Narrative

The purpose of this activity is for students to solve two-step story problems in the context of money using addition and subtraction. In this activity, each student starts with \$1 and buys multiple items. Students need to think about \$1 as 100¢ in order to solve each problem. The first problem is scaffolded, asking students to first find the cost of a pencil box and colored pencils and then find how much Clare will have after buying those items. For the other two problems this scaffold is removed. Students may observe, however, that Andre is buying the two items that Clare bought so they could use their calculation of the items Clare bought to help solve this problem.

When students connect the quantities in the story problem to an equation, they reason abstractly and quantitatively (MP2).

• Groups of 2

### Activity

• “Now you will solve a few more problems on your own and then compare with a partner.”
• 8 minutes: independent work time
• 2 minutes: partner discussion
• Monitor for students who use different methods for determining whether Andre has enough money.

### Student Facing

Show your thinking for each problem.

supplies cost
notebooks 26¢
colored pencils 18¢
pencil box 39¢
glue stick 44¢
1. Clare wants to buy a pencil box and colored pencils.

1. How much money will it cost?
2. Clare has $1. How much will she have left? 2. Tyler wants to buy a notebook and a pencil box. He has$1. How much will he have left?
3. Andre has \$1. He wants to buy a glue stick, a pencil box, and colored pencils. Does Andre have enough money?

### Activity Synthesis

• Invite previously identified students to share how they determined if Andre had enough money.
• As time permits, invite students to share how they found how much money Clare and Tyler had left.

## Lesson Synthesis

### Lesson Synthesis

“Today you solved story problems using addition and subtraction. In the second activity, each student had 1 dollar. How did you think about a dollar to solve the problems?” (Since we had to subtract the total number of cents, we needed to think about 1 dollar as 100 cents so we could subtract from 100.)