Stage: 3 Challenge Level: 
How does the sum of the numbers at the edges relate to the sum of the numbers at the vertices?
It might help to label the numbers at the vertices $A$, $B$ and $C$ and then express the edge numbers in terms of $A$, $B$ and $C$.
Games. smartphone. Interactivities. Workshop. Making and testing hypotheses. Creating expressions/formulae. Visualising. Simultaneous equations. Generalising. Mathematical reasoning & proof.
Published March 2005,April 2005,March 2011,April 2011.
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