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Find the Difference

Age 5 to 7
Challenge Level Yellow starYellow star
Primary curriculum
  • Problem
  • Getting Started
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Find the Difference

Find the Difference printable sheet

Place the numbers 1 to 6 in the circles so that each number is the difference between the two numbers just below it.

 

One circle in the top row, two circles in the next row, three circles in the bottom row

For example: The number 3 above the numbers 2 and 5  5 - 2 = 3

You could use this interactivity to try out your ideas:

 


Can you find more than one solution?

Can you find all the possible solutions? How do you know you have found them all?

 

Why do this problem?

This problem is a challenging way of practising subtraction at the same time as being logical about arranging the numbers. The idea of 'difference' can be hard for children to grasp and this problem is an ideal way of coming to terms with it. You could also use this problem to focus on how children record their workings.

Possible approach

You could start by putting the numbers in any places in the pyramid and asking children to describe what they see. They may notice some accidental number patterns, but also which numbers are used. Then put two numbers into the pyramid, for example 4 and 5 in the bottom row, next to each other. Introduce the idea of the problem and invite pupils to suggest what number should go above the 4 and 5. You could repeat this a few times with different pairs of numbers so that children are happy with 'difference' and with the way the pyramid will be structured. You may need to emphasise that the smaller number will be taken away from the larger number each time.

Introduce the task as stated in the problem and encourage learners to work in pairs. They may want to use the interactivity on computers and/or they could work on paper, using this sheet of blank pyramids for rough working and recording. After a short time, it would be useful to encourage learners to share the ways they are working and what they are recording. Some might be trying numbers then adjusting them, others might have thought about where, for example, the largest must go, others might be taking one number at a time and looking at the different places it could go in turn. The key point to make is that their recording system should somehow enable them to keep track of what arrangements they have tried so far so they don't repeat themselves. It is an advantage, therefore, not to use an eraser!
 
The plenary can be used to compare different solutions, using the interactivity, and to discuss the advantages of the different recording methods. Can any pair convince the rest of the class that they have found all the possible solutions?


Key questions

Where could the largest number go? Why?
What do we need to do to find the difference between two numbers?
How will you keep track of what you have tried?


Possible extension

Some learners might like to try Difference, which is much more challenging and uses the numbers from 1 - 10.

Possible support

Many children would benefit from using numbered counters that can be moved about if they don't have access to the interactivity. Some children may need to use manipulatives to support them in finding the differences.
 

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  • Back to The Problem with Problem Solving

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The NRICH Project aims to enrich the mathematical experiences of all learners. To support this aim, members of the NRICH team work in a wide range of capacities, including providing professional development for teachers wishing to embed rich mathematical tasks into everyday classroom practice.

NRICH is part of the family of activities in the Millennium Mathematics Project.

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