Wee hours

Algebra Level 5

Shortly after 3 o'clock, I look at a clock. The hands of the clock, listed clockwise, are in the order: hour-minute-second.

Moreover, the angle between the hour hand and the minute hand is the same as the angle between the minute hand and the second hand.

How many seconds after 3 o'clock is it? Round off to the nearest integer . Find the smallest positive solution.


The answer is 1038.

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3 solutions

Michael Mendrin
Aug 15, 2016

Kind of an infuriating problem. Start with a clock at exactly 3 : 00 3:00 . Let m m be the minutes from start, and M 1 , M 2 , M 3 {M}_{1}, {M}_{2}, {M}_{3} be the positions of the hour, minute, and second hand respectively. Then

M 1 = 15 + m 12 { M }_{ 1 }=15+\dfrac { m }{ 12 }
M 2 = m { M }_{ 2 }=m
M 3 = 60 ( m n ) { M }_{ 3 }=60\left(m-n\right)

where n n is the integer part of the minutes elapsed. We solve for m , n m, n the equation, for the first instance where both sides > 0 >0

M 2 M 1 = M 3 M 2 { M }_{ 2 }-{ M }_{ 1 }={ M }_{ 3 }-{ M }_{ 2 }

with the solution m = 17.3027... m=17.3027... and n = 17 n=17 , and the number of seconds is 60 m = 1038.16... 60m=1038.16...

Arjen Vreugdenhil
Aug 13, 2016

The positions of the hands of the clock can be represented as h h , m / 5 m/5 , and s / 5 s/5 , respectively, where one revolution corresponds to 12 units.

We know that m = 60 ( h H ) s = 60 ( m M ) m = 60(h-H)\ \ \ \ s = 60(m-M) where H = h H = \lfloor h\rfloor and M = m M = \lfloor m\rfloor are positive integers.

The problem describes the situation in which s / 5 m / 5 = m / 5 h ( = d > 0 ) . s/5 - m/5 = m/5 - h \ \ \ (= d > 0). Substituting the expression for s s , we get 12 ( m M ) m / 5 = m / 5 h 11 3 5 m + h = 12 M ; 12(m-M) - m/5 = m/5 - h\ \ \ \therefore\ \ \ 11\tfrac35 m + h = 12M; substituting for m m this becomes 11 3 5 60 ( h H ) + h = 12 M 697 h = 12 M + 696 H . 11\tfrac35\cdot 60(h-H) + h = 12 M\ \ \ \therefore\ \ \ 697h = 12 M + 696 H. Thus h = 12 M + 696 H 697 , m / 5 = 144 M 12 H 697 , s / 5 = 276 M 720 H 697 , h = \frac{12M + 696 H}{697},\ \ \ m/5 = \frac{144M - 12H}{697},\ \ \ s/5 = \frac{276M - 720 H}{697}, and it easy to check that the angle between the hands is d = s / 5 m / 5 = m / 5 h = 132 M 708 H 697 . d = s/5 - m/5 = m/5 - h = \frac{132M - 708 H}{697}.

Since it is shortly after 3 o'clock, we have H = 3 H = 3 . For M M we need the smallest value that makes d d positive. The inequality becomes d > 0 132 M > 708 H M > 5 4 11 H , d > 0 \ \ \ \therefore\ \ \ 132 M > 708 H \ \ \ \therefore M > 5\tfrac4{11}H, in this case, M > 16 1 11 M > 16\tfrac1{11} . Therefore the smallest solution after 3 o'clock is found when M = 17 M = 17 . We calculate h = 12 17 + 696 3 697 = 3.28838 ; h = \frac{12\cdot 17 + 696\cdot 3}{697} = 3.28838; subtracting 3 and multiplying by 3600 seconds per hour we find that this is 0.28838 3600 = 1038 0.28838\cdot 3600 = \boxed{1038} seconds after 3 o'clock.

Anubhav Tyagi
Aug 21, 2016

It was a nice problem... Really hats off to you man

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