Painted Purple

Three faces of a $3 \times 3$ cube are painted red, and the other three are painted blue. How many of the 27 smaller cubes have at least one red and at least one blue face?
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Problem



A wooden cube has three of its faces painted red and the other three of its faces painted blue, so that opposite faces have different colours.

It is then cut into $27$ identical smaller cubes.

How many of these new cubes have at least one face of each colour?

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