Show Me the Money! Two-Digit Subtraction
Students will be able to subtract by tens using number lines.
- Say, “Imagine we just won a district contest for the class with the most perfect attendance. The principal told me we earned a party! Now all we have to do is plan the party and make sure we have enough money to purchase all the supplies. Are you ready to help me spend some money?”
- Read the student objective and have them provide examples of Two-digit numbers(one whole number in both the ones and tens place), subtracting, and number lines.
Explicit Instruction/Teacher modeling(10 minutes)
- Explain to students that today they will be subtracting a lot of numbers using different place value strategies and number lines.
- Show students $30 in tens and $30 in ones.
- Ask students to think about the following problem. Say, "I want to know how much money I will have left if I spend $20 of my $30. Should I subtract $20 from the ones pile or $20 from the tens pile?"
- Ask, "Which would be better — to take away 10 at a time or to take away one at a time?"
- Direct students to turn and talk to a partner. (Tip: Look for answers that focus on arriving at an answer more quickly, fewer opportunities for mistakes, and show student understanding of place value.)
- Have students share their ideas with the whole class.
- Explain that we will subtract using the tens place in the place value chart so that we can subtract more quickly. Remind students that this is a good strategy to use to easily add and subtract.
- Project Sample Problem 1 with the document reader.
- Model how to subtract the two-digit numbers using a number line and incorporating the Add or Subtract by 10 on a Number Line Checklist.
- Determine three tens in $30.
- Draw the number line starting with $50.
- Skip-jump backwards $10 three times.
- Write the equation: $50 - $30 = $20.
Guided practise(15 minutes)
- Ask students to copy the problem work for Sample Problem 2 on their whiteboard as you solve it with them.
- Check off steps as you work, ask them for the next steps, and make some mistakes that require them to help you.
- Provide many examples of the supplies bought and the money spent (i.e. $60 - $30 for party prizes, $90 - $80 for entertainment).
- Say, “I want you and your partner to calculate how much money is left over after we buy our beverages.”
- Ask students to solve Sample Problem 3 with their partner and a whiteboard. Remind them to follow the checklist.
- Have a student share the answer and another student explain the method used to solve the problem.
- Subtract a few more problems together (for example: 90 - 60, 60 - 20, 30 - 10).
Independent working time(10 minutes)
- Have a few students review the Add or Subtract on a Number Line Checklist with the class.
- Distribute the Here Comes the Ten Monster! worksheet and read the directions with the students.
- Ask students to use the checklist and extra paper to draw the number lines.
- Circulate and monitor use of the number line, place value understanding, and answers.
- Use smaller two-digit numbers (10–20).
- Utilize pictures and the number line to represent the total amount, amount subtracted, and the leftover amount.
- Provide a place value chart or 100s chart for checking answers.
- Use bound sticks to represent tens and loose sticks for ones on a large place value chart for their adding or subtracting.
- Challenge students to subtract from 100.
- Students can create questions for their fellow students to solve.
- Have students complete Exit Ticket: Cake Money! to serve as formative assessment.
- Observe students during group work.
- Monitor students' proficiency as they complete the Add or Subtract on a Number Line Checklist.
- Collect the Here Comes the Ten Monster! worksheet.
Review and closing(2 minutes)
- Distribute Exit Ticket: Cake Money!
- Review the format and ask them to solve the problem.
- Remind students to use their Add or Subtract on a Number Line Checklist.