Chequered cuboid

A large cuboid is made from cubes of equal size. What fraction of the surface area of the large cuboid is black?
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Problem



Image
Chequered Cuboid
A cuboid is made from cubes of equal size, coloured alternately white and black as shown.

What fraction of the surface area of the cuboid is coloured black?

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