Tick Tock (More Algebra Than Geometry 3)

Algebra Level 3

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 Hours:Minutes:Seconds

If S e c o n d s Seconds can be written as a b \frac{a}{b} for co-prime natural numbers a a and b b , what is H o u r s + M i n u t e s + a + b Hours+Minutes+a+b ?

If S e c o n d s Seconds is irrational, let x x be S e c o n d s Seconds rounded to the nearest whole number. What is H o u r s + M i n u t e s + x Hours+Minutes+x ?

Note: Do not use a 24-hour clock.

Part 1

Part 2

Honorable Mention


The answer is 317.

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2 solutions

Chris Lewis
Apr 29, 2019

Over a period of 12 12 hours, the hands align exactly 11 11 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 12 11 \frac{12}{11} hours. Expressed in H:M:S, this is 1 : 5 : 300 11 1:5:\frac{300}{11} , giving the answer 317 \boxed{317} .

NumberDecompose [ 43200 11 , { 3600 , 60 , 1 } ] { 1 , 5 , 300 11 } 317 \text{NumberDecompose}\left[\frac{43200}{11},\{3600,60,1\}\right]\Rightarrow \left\{1,5,\frac{300}{11}\right\}\Rightarrow 317

A Former Brilliant Member - 2 years, 1 month ago
Jesse Li
Apr 27, 2019

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 m . The minute hand travels 6 6^\circ every minute, and the hour hand travels 1 12 \frac{1}{12} that speed, because it travels 5 minute marks every 60 minutes. So, the hour hand travels 0. 5 0.5^\circ 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 30^\circ , we can form the equation 6 m = 30 + 0.5 m 6m=30+0.5m . Solving gives m = 30 5.5 m=\frac{30}{5.5} , or m = 60 11 m=\frac{60}{11} , which is m = 5 5 11 m=5\frac{5}{11} as a mixed number.

So, 5 5 11 5\frac{5}{11} minutes need to pass after 1:00 before the hands point in the same direction. The 5 11 \frac{5}{11} minutes represents the number of seconds. Since there are 60 seconds in a minute, 5 11 \frac{5}{11} minutes is equivalent to 300 11 \frac{300}{11} seconds.

So, the result is 1 : 05 : 300 11 1:05:\frac{300}{11} . Now that we know that S e c o n d s Seconds can be written as a b \frac{a}{b} for co-prime natural numbers a a and b b , the answer is 1 + 5 + 300 + 11 = 317 1+5+300+11=\boxed{317}

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