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What's the time, Mr Caesar? (Posted on 2004-05-16) Difficulty: 2 of 5
I own a very unique clock. It is digital, but it displays the time in Roman numerals. It does this in the same way as a normal digital clock (hours : minutes). The clock does not display seconds. For example, at 7:34 the clock displays "VII:XXXIV". Whenever a zero is required it is represented by a dash (-). That is, 1:00 is represented by "I:-".

Sometimes I place this clock on a flat, horizontal mirror which reflects what is displayed on the clock.

For what proportion of the day does the reflection of the time on the clock look the same as the time displayed by the clock itself?

Please provide an answer for both 12hr timing (the clock does not indicate the difference between AM and PM) and 24hr timing.

  Submitted by Popstar Dave    
Rating: 2.3333 (3 votes)
Solution: (Hide)
This question is asking what proportion of times can be flipped vertically and still remain the same.

The symbols that this clock uses are:
- = 0 = symetrical
I = 1 = symetrical
V = 5 = non-symetrical
X = 10 = symetrical
L = 50 = non-symetrical

An analysis of the minutes section shows that 19 of the 60 minutes are symetrical. Basically, any number that doesn't contain a V or an L.
This breakdown makes it more obvious:
0-9 - 5/10 symetrical
10-19 - 5/10 symetrical
20-29 - 5/10 symetrical
30-39 - 4/10 symetrical
40-49 - 0/10 symetrical
50-59 - 0/10 symetrical

As for the hours section, in 12hr mode the clock displays the numbers 1 through 12, with 7 of these 12 being symetrical. In 24hr mode the clock displays the numbers 0 through 23, with 14 of the 24 being symetrical (note that unlike 12hr mode, the clock displays "-" for 0, which is symetrical, instead of "XXIV" for 24, which isn't).

In order for a time to be symetrical, both the hour and minute section need to be symetrical. Therefore the proportion of symetrical times is simply the product of the proportion of symetrical hours and proportion of symetrical minutes.

For 12hr mode:
(19/60) * (7/12) = (133/720)
                         = 18.47% (2d.p.)
For 24hr mode:
(19/60) * (14/24) = (266/1440)
                          = 18.47% (2d.p.)

P.S. I don't really own this clock, but I wish I did. It would be really confusing to read, but it would make a great conversation piece!

Comments: ( You must be logged in to post comments.)
  Subject Author Date
Puzzle ThoughtsK Sengupta2023-04-11 23:43:05
First tryThird2006-05-19 19:42:43
re: My trynikki2004-06-03 18:54:22
My tryIain2004-05-17 07:41:49
re: SolutionPopstar Dave2004-05-17 01:35:41
SolutionSolutionLarry2004-05-16 10:58:29
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