Subtracting Decimals: Presidential Problems at the Pizza Place
Students will calculate differences using decimal numbers to the tenths and hundredths.
- Ask students how they know if a number is more than or less than one. The purpose is to get them thinking about partial numbers and how we write them. They may provide answers that suggest fractions or decimals.
- Now ask students how they know if a decimal number is less than one.
- Write the number 14.36 (or something similar) on the board. Ask students which part of that number is less than one.
- Emphasize that the decimal is a very important marker that is boundary between whole numbers and numbers less than one whole.
- Inform students that they will be reviewing how to add and subtract decimals to the hundredths by calculating the meal totals of some famous politicians.
Explicit Instruction/Teacher modeling(10 minutes)
- Discuss how decimal numbers are similar to our system of money. Point out that they are both base ten, so the place values increase by multiples of 10 as you move to the left and decrease by multiples of 10 to the right.
- Review place value from the thousandths down to the hundredths. Explain that a one in the hundredths place is equal to one cent (1/100 of a dollar), and a one in the tenths is equal to a dime (1/10 of a dollar), and so on.
- Tell students that the decimal is the most important thing to pay attention to when subtracting (and adding) decimals.
- Model subtraction of decimal numbers on the board for the class. Explain that it is the same as any other subtraction but they need to first line up the decimals. Start with numbers to the tenths, like 14.5 - 6.9, then show an example of decimals to the hundredths.
Guided Practice(10 minutes)
- Distribute the Presidential Problems at The Pizza Place! worksheet.
- Read the instructions together and discuss the first problem. Model identifying pertinent information (by circling or underlining) and showing calculations in the space provided.
- Write the answer in a complete sentence.
- Have students complete number two on their own or with a partner.
- Review the process and solution.
Independent working time(20 minutes)
- Instruct students to complete the next two problems independently and then create two of their own subtraction problems using the menu.
Support: Allow students to check their answers with a calculator after each one and revisit any errors in their process with your or a friend.
- Enrichment: Challenge students to design menu problems also using division and multiplication.
- Pose the question, "What is the difference between Barack and Michelle’s total and Donald and Melania’s total?"
- Have students write the problem (showing their work) and their solution on a piece of scratch paper. Either spot check or collect.
Review and closing(5 minutes)
- Call on a few students to share aloud the problems they authored as teacher challenges. Have students read the problem while you (or another adult or students in the room) solve it in front of the class.
- Discuss, "What was challenging about this activity? What strategies did you use to deal with that challenge? Did they work?"