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'Finding Factors' printed from http://nrich.maths.org/

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The interactivity below is similar to the one used in Missing Multipliers so you may wish to work on that first.

In the multiplication grid below, the headings and the answers have been hidden. Each of the headings is an expression of the form $x \pm a$ where $a$ is an integer between $0$ and $5$. By revealing some of the answers, can you work out what each heading must be? 
 
Drag the green and purple labels onto the headers to make the correct expressions. If you change your mind, you can remove a label by clicking on it.

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What is the smallest number of answers you need to reveal in order to work out the missing headers?
Can you describe a strategy for completing the Level 3 challenge consistently?
Once you have a strategy for completing Level 3, try this slightly more challenging version:
 
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