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This is written as a follow-on from Build it Up.

You could extend what you did in Build it Up by thinking of it in 3D!

The circles would then be spheres.
Each numbered sphere would then sit on the three underneath and the whole thing would end up being a triangular based pyramid.

So there would be six numbers (in an equilateral triangle) at the bottom, as in this example:

Then the next layer up would be:
 
So the top layer would be:
 
 
You can explore finding a triangle of six numbers (we don't use zero!) and add them in threes to give a triangle of three numbers. As in Build it Up, we would then like those three numbers to add to 15 at the very top of the pyramid.

Here's a pdf version of this activity: BuildItUpMore.pdf