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Pentagonal Time (Posted on 2022-01-30) Difficulty: 2 of 5
The current time is between 1 o'clock and 12 o'clock in a wall clock and both the minute hand and the hour hand are situated precisely on two distinct minute marks.

It is observed that the hour hand is precisely a pentagonal number of minute marks ahead of the 12 hour mark. The number of minute marks by which the hour hand is ahead of the minute hand is also a pentagonal number greater than 1.

Determine the current time from the information provided in the above statements.

No Solution Yet Submitted by K Sengupta    
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soln, now | Comment 4 of 10 |
Thanks for the fix. The answer is 4:12 with the minute hand 51 marks ahead. The series of new differences is 60 minus the old differences: (M - H) = 60 - (H - M).  The (M-H) differences are:
01:12  6
02:12  1      (As in the recent problem "Time after Time")
03:12 56
04:12 51
05:12 46
06:12 41
07:12 36
08:12 31
09:12 26
10:12 21
11:12 16
12:12 11 (forbidden),
with 51 being the only pentagonal number listed. Yes, algebra with 
functions of hour h, equating (M-H) and p_h= (3h^3-h) / 2 would prove this, but why bother?

Edited on January 30, 2022, 4:20 pm
  Posted by Steven Lord on 2022-01-30 15:55:47

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