Minds On

Mental math subtraction

How can you use mental math to subtract?

For example, how would you calculate 13 − 8 = ?

We can subtract in two ways: by taking away (begin with 13 and take away 8) or by counting up (begin with 8 and count up to 13). Solve the subtraction question using either way.

Let’s explore some tools that we can use to help subtract:

We can use:



Number lines

Number lines from one to twenty

Mental math

Person thinking


Activity – Tracking water usage

A student noticed that their family used 88 litres of water in a week. The student and their family made a plan to reduce their water usage by 5 litres each week. How might they reduce their use of water?

Keeping all of this in mind, the student created the following chart to track the use of water per week.

Number of Weeks Litres of water used that week
Week 1 88
Week 2 83
Week 3
Week 4
Week 5
Week 6
Week 7
Week 8
Week 9
Week 10

Can you help the student keep track of how much water their family has used by filling in their chart? The family’s goal is to reduce their water usage by 5 litres per week.


  • How do you know your answers are correct?
  • What tool(s) helped you get your answers?
  • At which week would Zoe and her family’s water use reach 68 Litres?
  • If the student and their family continued to reduce the amount of water they used, at which week would they reach 53 litres of water used?


Solving a subtraction problem

A family of beavers

Beavers like to eat bark and stems. Mother Beaver is collecting food for their kits (Hint: a kit is a baby beaver) Mother collects 78 logs in total. The first kit eats 25 logs, and the second kit eats 15 logs. How many logs are left for the third kit?

Please record how you solved the problem. Include a number sentence for each of your steps.


How do you feel about what you have learned in this activity?  Which of the next four sentences best matches how you are feeling about your learning? Press the button that is beside this sentence.

I feel...

Now, record your ideas about your feelings using a voice recorder, speech-to-text, or writing tool.