At 12:00:00, the hour and minute hands of an analog clock point in the same direction. What is the next time the hour and minute hands point in the same direction?
The answer can be written in the form H o u r s : M i n u t e s : S e c o n d s
If S e c o n d s can be written as b a for co-prime natural numbers a and b , what is H o u r s + M i n u t e s + a + b ?
If S e c o n d s is irrational, let x be S e c o n d s rounded to the nearest whole number. What is H o u r s + M i n u t e s + x ?
Note: Do not use a 24-hour clock.
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NumberDecompose [ 1 1 4 3 2 0 0 , { 3 6 0 0 , 6 0 , 1 } ] ⇒ { 1 , 5 , 1 1 3 0 0 } ⇒ 3 1 7
The time has to be a little after 1:00, because the minutes hand needs to make more than a full revolution before it can point in the same direction as the hour hand.
At 1:00, the hour hand points at the 5 minute mark. We can call the amount of minutes that need to pass before the hands point in the same direction m . The minute hand travels 6 ∘ every minute, and the hour hand travels 1 2 1 that speed, because it travels 5 minute marks every 60 minutes. So, the hour hand travels 0 . 5 ∘ every minute. Because the hands need to point in the same direction and the hour hand is ahead by 5 minutes, which is 3 0 ∘ , we can form the equation 6 m = 3 0 + 0 . 5 m . Solving gives m = 5 . 5 3 0 , or m = 1 1 6 0 , which is m = 5 1 1 5 as a mixed number.
So, 5 1 1 5 minutes need to pass after 1:00 before the hands point in the same direction. The 1 1 5 minutes represents the number of seconds. Since there are 60 seconds in a minute, 1 1 5 minutes is equivalent to 1 1 3 0 0 seconds.
So, the result is 1 : 0 5 : 1 1 3 0 0 . Now that we know that S e c o n d s can be written as b a for co-prime natural numbers a and b , the answer is 1 + 5 + 3 0 0 + 1 1 = 3 1 7
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Over a period of 1 2 hours, the hands align exactly 1 1 times (just think about how many times the minute hand sweeps past the hour hand each hour). The time between consecutive alignments is always the same; so it must be 1 1 1 2 hours. Expressed in H:M:S, this is 1 : 5 : 1 1 3 0 0 , giving the answer 3 1 7 .