WOLFRAM|DEMONSTRATIONS PROJECT

Two-Cube Calendar

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click for a problem
loosen cubes
apart
What is the (minimal) sum of the four digits that cannot be seen on the left
cube?
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Answer!
11
True

Each face of two cubes is labeled with a single digit from 0 to 9. It is possible to show any date from 01 to 31 on the front faces. The problem is to determine the sum of the digits that cannot be seen.

See the details for a hint.