Christmas Streak 64/88: Clock #3

Logic Level 5

Let the angle formed by the hour and minute hands of a clock be α , \alpha ^{\circ}, where α 180. \alpha\le 180.

During a day, how many times does α \alpha become a positive integer?


Details and Assumptions:

  • A day is 24 hours.
  • The minute and hour hands of the clock move perfectly continuously.
  • The measurement of the angle formed by the two hands is done on the smaller angle.

This problem is a part of <Christmas Streak 2017> series .


The answer is 7898.

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2 solutions

Miles Koumouris
Dec 5, 2017

In a given day, the minute hand does 24 24 full rotations, and the hour hand does 2 2 full rotations, meaning that the two hands line up 24 2 = 22 24-2=22 times in a day. From one instance when the hands line up to the next instance, exactly 360 1 = 359 360-1=359 positive, integral values of α \alpha will be attained. Hence, the answer is 22 × 359 = 7898 22\times 359=\boxed{7898} .

Patrick Corn
Dec 5, 2017

Starting at midnight, after m m minutes, the minute hand has swept out 6 m 6m degrees, and the hour hand has swept out m / 2 m/2 degrees. So α \alpha is an integer if and only if 6 m m / 2 = 11 m / 2 6m-m/2=11m/2 is an integer. (Note: I'm not saying that α = 11 m / 2 , \alpha = 11m/2, just that they are either both integers or both not.)

There are 1440 1440 minutes in a day, so at midnight of the next day, 11 m / 2 = 7920. 11m/2 = 7920. So 11 m / 2 11m/2 takes every integer value between 0 0 and 7920 7920 once in that 24 24 -hour period. But we must throw out the values where 11 m / 2 11m/2 is a multiple of 360 , 360, since these correspond to α = 0 \alpha = 0 and we want α \alpha to be a positive integer. So we throw out 0 , 360 , 720 , , 7920. 0,360,720,\ldots,7920. Our final answer is just 7920 7920 / 360 = 7898 . 7920 - 7920/360 = \fbox{7898}.

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