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

Example:   $5 - 2 = 3$

You could try it in this interactivity.

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 $10$

Here is an alternative Flash interactive.
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