When Subtraction becomes Addition
Solve subtraction problems by using addition.
Number Framework Stage 6
Using Materials
Problem: “Ana goes Christmas shopping. She spends $345.60 on toys for her children and comes home with $208.95. How much money did she have to start with?”
Write ? – 345.60 = 208.95 on the board. Have the students cut a strip of paper, representing Ana’s unknown starting amount.
Cut off a piece and write $345.60 on it.
Discuss what goes on the other piece. (Answer: $208.95.)
Discuss what the starting amount must have been. (Answer: $345.60 + $208.95 = $554.55.)
Examples: Word stories and recording for: ? – 345 = 789
? – 456 = 102
? – 67.9 = 43.19
? – 67 = 90
? – 1 000 = 3 000
? – 200 = 560 ...
Using Imaging
Examples: Imagine the strips to solve these problems: Word stories and recording for:
? – 200 = 600
? – 220 = 230
? – $7.90 = $3.10
? – 111 = 238
? – 10 000 = 4 500 ...
Using Number Properties
Examples: Don’t use calculators for the fraction problems. Word stories and recording for:
? – 212.98 = 600.0034
? – 200.08 = 45.89
? – 7.9909 = 3.1091
? – 10 = 3/4
? – 2 1/3 = 1/3
? – 0.04 = 0.0009 ...
Understanding Number Properties:
a and b stand for any numbers: â–¡ – a = b. How would you work out
the number to put in the box? (Answer: I would work out the answer to a + b.)
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