Snack Shop Addition and Subtraction

Est. Class Sessions: 1–2

Developing the Lesson

Review Subtraction Strategies. Display and direct students’ attention to the Subtraction Strategies Chart from Lesson 2 and the Subtraction Strategies Menu from the Student Activity Book Reference section. Write the following three problems on the board:

$1.75	98¢	$1.51
− $1.25	− 59¢	− .30

Ask:

Which of the problems might you do:
in your head?
with base-ten pieces?
using the 200 Chart?
using a number line?
with paper and pencil?

Have students explain their thinking and demonstrate how they would solve one of the problems a particular way. Students may respond that they can solve $1.75 − $1.25 in their heads because they think about quarters and because no trades need to be made. Because there are trades to be made, some students may prefer to use base-ten pieces or paper-and-pencil strategies to solve 98¢ − 59¢. Some students might use a compensation strategy: 100¢ − 59¢ = 41¢, 41¢ − 2¢ = 39¢. For $1.51 − $.30, a student could start at 151 on the 200 Chart and move back three rows to subtract 30 or mentally subtract three tens from 151. Other students might start at 151 and take three hops of ten backwards on the number line.

Solve Addition and Subtraction Problems. Display and direct students to the Shooting Star Snack Shop Children’s Menu page in the Student Activity Book. Asking students to buy only one item and calculate the change allows them to practice some of the strategies they learned in this unit. Encourage students to share their strategies. Begin by posing the following situations to the students.

Ask:

Suppose you had 50¢ to spend. What item would you buy, and how much change would you get back? (Possible response: a brownie for 35¢; I would get 15¢ in change.)
What is a number sentence that describes the problem? (50¢ − 35¢ = 15¢)
What labels should you include so we know what the numbers in your answer mean? (cents, ¢)
How did you figure out how much change you were due? (Possible response: I counted up 5¢ to 40¢ and then 10¢ more to 50¢. 5¢ + 10¢ = 15¢.)
Suppose you really wanted a slice of pizza. What is the number sentence that describes how much more money you will need? (79¢ − 50¢ = )
How do you know how much more money you need? (29¢; I solved 79¢ − 50¢ in my head. I took 5 tens from 7 tens and 0 ones from 9 ones and got 2 tens and 9 ones. That meant I would need 29¢ more to buy pizza.)
Did anyone use a different strategy?
Suppose you put your money together with your friend. You now have $1.00 to spend and you buy peanut butter and crackers to share. Show or tell how to use mental math to find out how much change will you get. (Possible response: 51¢; I counted up 1 to 50, and then added another 50 to get 100.)
Let’s solve that same problem using a paper-and-pencil method. [See Figure 1.]
Which way do you prefer? Is one quicker or easier than the other? Is one more efficient? (Possible response: For this problem, it was easier and faster for me to use mental math.)

Next display the Snack Shop Sample Master. Read the problem on the top half of the Master and solve it as a class. This problem requires students to find the total cost of three items. First they must estimate the total.

Ask:

Pretend you have $1.25 to spend at the snack shop. Do you think you have enough money to buy carrot sticks, potato chips, and a small lemonade? Explain how to estimate the total cost. (Possible response: Rounding up on the 200 Chart to the nearest ten, I think 30 + 30 + 60 = 120 and I think I will have enough money.)
How could you add to find out if $1.25 is enough? (Add 29¢ plus 25¢ plus 55¢ and see if it is less
than $1.25.)
How could you subtract to find out if $1.25 is enough? (Start with $1.25. Subtract 29¢, then subtract 25¢, and then subtract 55¢ to see if you still have
money left.)

Have students explain their thinking and demonstrate how they would solve the problem. Discuss more than one method. Some may use mental math, while others choose to use tools such as the 200 Chart, number line, or base-ten pieces. Still other students may use paper and pencil.

Ask:

How much change should you receive? Show or tell how you know. (16¢; Solution strategies for
$1.25 − $1.09 will vary.)
How can you use a calculator to check your calculations? (I can check my addition by calculating 29 + 25 + 55. I can check my subtraction by calculating 125 − 109. Or, I can check my subtraction by adding
16 + 109 to see if I get 125.)

Finally, direct students’ attention to the form on the bottom half of the Snack Shop Sample Master.

Ask:

Pretend you have $2.00 to spend at the snack shop. What will you order? Remember to estimate as you choose so that you do not go over $2.00.

Have students work on the problem with a partner, finding the total cost and the amount of change due for their order. Then, as a class, design one order to list on the form on the Snack Shop Sample Master. Work through the problem together using the following steps:

Choose the items from the menu.
Estimate whether there is enough money.
Calculate the total.
Find the amount of change due.

Encourage students to share their strategies for solving addition problems with more than two addends. Have students also share their subtraction strategies. Remind them to include dollar or cent signs in their answers. Demonstrate how to use a calculator to check the calculations.

Assign the problems on the Snack Shop Bills pages in the Student Activity Book. Encourage students to refer to the Addition Strategies Menu and the Subtraction Strategies Menu in the Student Activity Book Reference section and the Subtraction Strategies Chart from Lesson 2. Provide access to the tools they have used in class.

Share Solution Strategies. Upon completion, provide an opportunity for students to share their solutions for Question 5 on the Snack Shop Bills pages. Talk about estimation, addition, and subtraction strategies. Try to choose volunteers who can share a variety of strategies including some paper-and-pencil and mental math strategies. Use calculators to check students’ subtraction with addition.

Questions to ask include:

How did you estimate to help you know if you had enough money to buy the items you wanted? (Possible response: I estimated the cost of my items by adding the tens. I realized I had spent too much money so I chose a less expensive item.)
How did you find the sum?
How did you find out how much change you
were due?
Which tools did you use? (Possible response:
200 Chart, calculator, number line, base-ten pieces, Subtraction Strategies Menu)

Use Check-In: Question 6 and the Feedback Box on the Snack Shop Bills pages in the Student Activity Book to assess students’ abilities to add and subtract using mental math strategies [E3]; add and subtract using expanded form and the compact method [E4]; know the problem [MPE1]; choose good tools and efficient strategies [MPE2]; show work [MPE5]; and use labels to show what numbers mean [MPE6].

Adventure Book: