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Two digit cubes that show every date of a month

Question

You have two blank cubes and may write one digit on each of their twelve faces. Placed side by side, the cubes must be able to display every day of a month as a two-digit number, from 0101 up to 3131. How should the faces be labelled?

Short answer

Cube one: 0,1,2,3,4,50,1,2,3,4,5; cube two: 0,1,2,6,7,80,1,2,6,7,8, with the 66 turned upside down to serve as 99. Dates 1111 and 2222 force 11 and 22 onto both cubes, and 0101–0909 force 00 onto both as well, leaving only six free faces for the seven digits 33–99. The trick is that one face can read as either 66 or 99.

Adapted from Zhou, A Practical Guide to Quantitative Finance Interviews, sec. 2.3 Thinking Out of the Box, 'Calendar cubes', p.10; question and answer rewritten by SelfMadeQuant.com.

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