Let the angle formed by the hour and minute hands of a clock be α ∘ , where α ≤ 1 8 0 .
During a day, how many times does α become a positive integer?
Details and Assumptions:
This problem is a part of <Christmas Streak 2017> series .
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Starting at midnight, after m minutes, the minute hand has swept out 6 m degrees, and the hour hand has swept out m / 2 degrees. So α is an integer if and only if 6 m − m / 2 = 1 1 m / 2 is an integer. (Note: I'm not saying that α = 1 1 m / 2 , just that they are either both integers or both not.)
There are 1 4 4 0 minutes in a day, so at midnight of the next day, 1 1 m / 2 = 7 9 2 0 . So 1 1 m / 2 takes every integer value between 0 and 7 9 2 0 once in that 2 4 -hour period. But we must throw out the values where 1 1 m / 2 is a multiple of 3 6 0 , since these correspond to α = 0 and we want α to be a positive integer. So we throw out 0 , 3 6 0 , 7 2 0 , … , 7 9 2 0 . Our final answer is just 7 9 2 0 − 7 9 2 0 / 3 6 0 = 7 8 9 8 .
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In a given day, the minute hand does 2 4 full rotations, and the hour hand does 2 full rotations, meaning that the two hands line up 2 4 − 2 = 2 2 times in a day. From one instance when the hands line up to the next instance, exactly 3 6 0 − 1 = 3 5 9 positive, integral values of α will be attained. Hence, the answer is 2 2 × 3 5 9 = 7 8 9 8 .