Don’t Subtract – Add!

Use the strategy “Jumping the number line” – book 5
Addition and subtraction facts to 20

This activity uses reversing as a strategy to solve subtraction problems, e.g. 51 - 34 = ? is the same as finding what you add to 34 to get to 51.
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).
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

 

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