Subtraction to Subtraction

Using Materials

Problem: “Joyce borrows $13,800 to buy a new car. At the end of the year, she still owes $7,890. How much of the loan has she paid?”

Write 13 800 – ? = 7 890 on the board. Get the students to make the following strips:

Discuss why the answer is 13 800 – 7 890 and then get the students to work out the answer on calculators. (Answer: 5 910.)

Examples: Word stories and recording for: 34 567 – ? = 5 678

12.45 – ? = 7.01
20 008 – ? = 13 856 ...

Using Imaging

Problem: “Caterers have 3 190 pies to sell at a rugby league game. At the end of the day, they have 345 pies left. How many pies were sold?”

Write 3 190 – ? = 345 on the board. Encourage the students to imagine the strips to solve the problem. (Answer: 3 190 – 345 = 2 845 pies.)

Examples: Word stories and recording for: 4 567 – ? = 2 888

102.45 – ? = 41.89
12 789 – ? = 5 692
23.456 – ? = 4.991 ...

Using Number Properties

Don’t use calculators for the fraction problems.
Examples: Work out: 88 001 – ? = 84 067

1 000 000 – ? = 890 023
0.003456 – ? = 0.0000312
10 3/7  – ? = 1 1/7
67/77 – ? = 3/77
3 1/2  – ? = 1 1/2
23  1/11– ? = 13 ...

Understanding Number Properties:

a – ? = b. What number goes in the box? old; (Answer: a – b is the number that goes in the box.)