We use cookies to personalise content, to provide social media features and to analyse our traffic. With your consent we share some information about your use of our site with our analytics partner who may combine it with other information that you have provided to them or that they have collected from your use of their services. You consent to use some cookies if you continue to use this website.

Required:  ✔ Essentials   ✔ Respecting Do Not Track

# Does it Matter - Subtraction

Published about 2 years ago by Ann Baker

Use this DOES IT MATTER slide to initiate conversations about what order to subtract and add these numbers.

We spend a lot of time learning how to read and write from left to right, in a set order. All of a sudden students are expected to look at a number sentence or question such as those presented in this DOES IT MATTER and decide whether to simply read left to right and carry out the operations in the order presented or to chunk particular parts together first.

In the examples given here it is important to know that the changing the order in which the two operations are worked out will create 2 different outcomes and to be able to explain why that is so. Many students come to understand that addition is commutative and then over generalise commutativity to questions such as the ones presented above.

We can see that each question has 2 possible answers depending on the order in which the operations are worked out in For 8 – 3 + 1

• Doing the subtraction first leads to 8 – 3 = 5 followed by 5 + 1 = 6
• Doing the addition first leads to 3 + 1 = 4 followed by 8 – 4 = 4

Students need time to explore and explain why this is so. More importantly though they need to match which interpretation to use in a particular situation. For example: a) l had \$8 in my purse. I spent \$3 and then found a dollar coin. How much money do I have now? as opposed to: b) I had \$8 in my purse. I spent \$3 on a notebook and \$1 on a pencil. How much money do I have left?

The next step is to be able to link the number sentence to match the intent: a) (8 – 3) + 1 b) 8 – (3 + 1)

A young student that I was working with once used brackets spontaneously without being taught. She held up her hands to demonstrate that she had chosen which operation to do first:

Maybe we over complicate things when we explain and should listen to and watch how students can come to make sense of things.

Allow time for students to compare answers and talk about what they notice and encourage them to use brackets to 'chunk' parts to match a word problem that they have created with the numbers and operations given.

Remember that these discussions are intended to lead to deepening understandings so it is important that students and teacher both listen to the ideas and reasoning of others and challenge thinking respectfully and constructively. Only under 'safe and respectful' conditions can students take a risk and explore ideas as they make mistakes, clarify thinking and test the validity of their reasoning.

This second DOES IT MATTER looks like the first one except that the questions only use subtractions. This time the order in which the operations are carried out doesn’t matter and using a collection of objects may help to make this clear.

Download a copy here, to use with your class if needed: Does It Matter 1 - subtraction-2

Check out our website for more blog articles and practical support. Looking to help your students with subtraction? Check out our Subtraction Mats or our Mental Routine on-line course