Super-imposed?

Geometry Level 2

When are the hour and minute hands of a clock superimposed?

When the angle of the hour hand is 0 0^\circ When their angles are the same When the angle of the hour hand is 9 0 90^\circ When the angle of the minute hand is 9 0 90^\circ When the angle of the minute hand is 0 0^\circ

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2 solutions

Ayush G Rai
Sep 10, 2016

The hour and minute hands are superimposed only when their angle is the same.
θ min = θ hr 6 × M = 0. 5 × ( 60 × H + M ) 12 × M = 60 × H + M 11 × M = 60 × H M = 60 11 × H M = 5. 45 × H {\displaystyle {\begin{aligned}\theta _{\text{min}}&=\theta _{\text{hr}}\\\Rightarrow 6^{\circ }\times M&=0.5^{\circ }\times (60\times H+M)\\\Rightarrow 12\times M&=60\times H+M\\\Rightarrow 11\times M&=60\times H\\\Rightarrow M&={\frac {60}{11}}\times H\\\Rightarrow M&=5.{\overline {45}}\times H\end{aligned}}}
H is an integer in the range 0 11. 0-11. This gives times of: 0 : 00 , 1 : 05. 45 , 2 : 10. 90 , 3 : 16. 36 , 4 : 21. 81 , 5 : 27. 27 . 6 : 32. 72 , 7 : 38. 18 , 8 : 43. 63 , 9 : 49. 09 , 10 : 54. 54 , 0:00, 1:05.\overline{45}, 2:10.\overline{90}, 3:16.\overline{36}, 4:21.\overline{81}, 5:27.\overline{27}. 6:32.\overline{72}, 7:38.\overline{18}, 8:43.\overline{63}, 9:49.\overline{09}, 10:54.\overline{54}, and 12 : 00. 12:00. ( 0. 45 (0.\overline{45} minutes are exactly 27. 27 27.\overline{27} seconds.)

Prince Loomba
Sep 10, 2016

This is an easy problem, we have to think what does superimpose mean, it means they are at same angle from a point. Other options are eliminated easily as for any option there is some angle between hour and minute hands given by 30 h 5.5 m |30h-5.5m|

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