Don’t Subtract – Add!
Teacher’s Notes:

These exercises and activities are for students to use independently of the teacher to practise number properties.

Number Framework domain and stage:

Addition and subtraction. Early additive to advanced additive

Curriculum reference:

Number, level 2 to3

Numeracy Project book reference:

These exercises and activities follow from a teaching episode based on Book 5 Page 34 and are for those students who are able to use the associated number properties.

Prior knowledge. Students should be able to…

During these activities students will meet:

Using addition to find the answer to subtraction problems (All exercises)
Converting word problems into number problems (Ex 2, 8)

Background:

The exercises have been set up in the following way.
Exercise 1: Requires students to rewrite subtraction as an addition problem and find the answer.
Exercise 2: Requires students to write the subtraction problem from the words, rewrite as an addition problem and find the answer.

Exercises 3 – 7 are all straight number problems requiring students to use an addition method to work out the subtractions.
Exercise 3: Both numbers are less than 100.
Exercise 4: First number between 100 and 200, second number near 100.
Exercise 5: Both numbers in same hundred.
Exercise 6: Numbers in different hundred, second number near a hundred.
Exercise 7: Numbers in different hundred.

Exercise 8: Requires students to convert the word problem to a number problem. They then need to choose the problems for which “don’t subtract – add!” would be a suitable strategy.

Note about word problems.
One issue with providing word problems in an exercise alongside number problems is that some students learn not to read the words, but to simply pull out the numbers and “do the same to them”. To start addressing this problem, exercise 8 has a mixture of addition and subtraction problems. The subtraction problems are a mix of those for which “don’t subtract – add!” would be suitable strategy and some for which it is not. For example - two of the subtraction problems are easier to do by doing a direct subtraction of the “tens” and then “ones” as the second number has both the “tens” and “ones” values smaller than the first number. This exercise also provides a good basis for a teaching session around what words tell us that we should be subtracting the numbers (or adding the numbers).

Homework Exercises:
Follow the same model as the practice exercises.

Number generalisation:
Ideas that can be developed from this activity:
63 – 29 = •; 29 + • = 63; • + 29 = 63; 63 - • = 29
a – b = •; b + • = a; • + b = a; a - • = b

72 – 38 = x; 38 + x = 72; x + 38 = 72; 72 – x = 38
a – b = x; b + x = a; x + b = a; a - x = b

Practice exercises with answers:

PDF (115KB) or Word (68KB)

Homework with answers:

PDF (112KB) or Word (58KB)

Related games/activities:

Fish or memory card game:PDF (73KB) or Word (39KB)

Fish master:Word (25KB)

Figure It Out References

Basic Facts – Level 3 – Page 8 Array Puzzles
Number Sense Book One – Year 7-8 – Page 17 Different Approaches