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'Find the Difference' printed from http://nrich.maths.org/
Place the numbers $1$ to $6$ in the circles so that each number is the difference between the two numbers just below it.
$5 - 2 = 3$
You could try it in this interactivity.
Full screen version
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When you have tried this and got more than one answer you could try other things like having $4$ rows of discs and use the number $1$ to $10$