A child's box of bricks contains cubes, cones and spheres. Two cones and a sphere on one side of a pair of scales will just balance a cube on the other side;

and a sphere and a cube together will just balance three cones.

How many spheres will just balance a single cone?

If you liked this problem, here is an NRICH task which challenges you to use similar mathematical ideas.

This problem is taken from the UKMT Mathematical Challenges.