Reversing Addition

Using Materials

Problem: “Murray has $1,858 in the bank. His grandfather put some more money in Murray’s account. Now Murray has $5,683. How much money did Murray’s grandfather put in Murray’s account?”

Write 1 858 + ? = 5 683 on the board. Get the students to make two strips of paper of equal length and label them as shown:

Discuss why 1 858 off the second strip shows the answer is 5 683 – 1 858 and work out the answer with a calculator.

Problem: “Geraldine collects stamps. Her parents give her a packet of 355 stamps for her birthday. Altogether she now has 6 040 stamps. How many stamps did Geraldine have before her birthday?”

Write ? + 355 = 6 040 on the board. Repeat the method with strips shown above.

Examples: Word stories and recording for: 3 333 + ? = 4 141

? + 5 601 = 45 893
? + 7 928 = 30 281
$234.56 + ? = $789.40 ...

Using Imaging

Problem: “Miles has $345, and he wants to buy a mountain bike costing $601. How much more money does he have to  save?”

Write 345 + ?= 601 on the board. Ask the students to imagine the strips and solve the problem. Drop back to drawing the strips on the board if needed.

Examples: Word stories and recording for: 4 567 + ? = 6 012

? + 9 567 = 12 211
? + 6 443 = 14 601
$291.36 + ? = $1,089.43 ...

Using Number Properties

Examples: Use a calculator to work out these answers: 44 675.83 +  ? = 76 645.93

? + 345.67 = 5 678.09
444 1/3 +  ? = 800 1/3
484 609 + ? = 1 678 980
? + 34.78902 = 56.00912 ...

Understanding Number Properties:

a + ? = b where a and b stand for numbers. How would you work out the number that goes in the box? (Answer: Work out the answer to b – a.)