Not true with subtraction facts: 8 – 5 = 3 and 8 – 3 = 5 appear as two separate subtraction facts. They are to a young student in the same fact family, but students must learn both of these subtraction facts. There is no commutative property of subtraction. To complicate matters, students must also learn double digit subtraction facts subtracting a single digit (13 – 8 = 5 and 13 – 5 = 8). Ask any elementary math teacher – the vast majority will agree. Subtraction facts are kids’ main adversary.
Is there a method to help kids to quickly learn many of their subtraction facts? Yes. There is!
Students still need to learn single digit from single digit subtraction facts (5 – 3 = 2 and 9 – 6 = 3), but those facts are much easier than single digit numbers subtracted from a double digit number (i.e. 17 – 9 = 8). There is a method that is very helpful in the latter case and adds numeracy skills of number sense and base 10 mathematics at the same time. This subtraction technique relies on student proficiency of the addition of two single digit numbers – a skill that is typically mastered well before working on subtraction facts.
This method is shown below using the math fact 15 – 7 = 8 with a typical subtraction fact format.
This method may appear complicated, but it has merit. After students are able to ‘Make 10’ quickly, they become very adept at the process – and subtraction facts are quickly mastered. The math skill of ‘Making 10’ should be taught to mastery prior to using this subtraction technique. Skill practice sheets for ‘Making 10’ are available in the Formative Loop Numeracy Resources Library.
Teachers should show the process on a number line so kids visually understand the physical nature of the mathematics and why this process ONLY works when subtracting a 1 digit number from a 2 digit number.