# Rubo Cubo

How many unit cubes are visible on a $$7\times 7\times 7$$ cube?

For example,

• the $$2\times 2\times 2$$ cube has 8 visible unit cubes;
• the $$3\times 3\times 3$$ cube has 26 visible unit cubes;
• the $$4\times 4\times 4$$ cube has 56 visible unit cubes;
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