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Cubic Covering

Age 14 to 16 Short
Challenge Level
We first imagine the cross with the blue cubes only:

We now glue the yellow faces to the up and bottom faces of the blue cross. We require 5 yellow cubes for wrapping each of the two blue cubes. (We used transparent yellow cubes)

Now we need one yellow cube for each of the four corners:
Finally, we have 4 faces to cover, so we need 4 more squares.

Therefore, we used $5+5+4+4 = 18$ yellow cubes.

This problem is taken from the UKMT Mathematical Challenges.
You can find more short problems, arranged by curriculum topic, in our short problems collection.