How many times a day do the minute and hour hands of an analog clock overlap exactly? (Don't count the overlap at 12:00 twice)
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The clock and its hands
Hints
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- The hour hand completes one revolution in 12 hours, while the minute hand completes one revolution in 1 hour. This means that the minute hand "advances" the hour.
- Angular speeds: Minute hand: 360° per hour = 6° per minute.
- In 24 hours: 11 × 2 = 22 overlaps (not counting the one at 12:00 twice).
Solution
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Answer: 22 times a day.
Explanation:
The hour hand completes one revolution in 12 hours, while the minute hand completes one revolution in 1 hour. This means that the minute hand "advances" the hour.
Angular velocities:
- Minute hand: $360°$ per hour = $6°$ per minute
- Schedule: $30°$ per hour = $0.5°$ per minute
- Relative speed: $6 - 0.5 = 5.5°$ per minute
For the minute hand to reach the hour from one overlay to the next, it must gain $360°$:
$$\text{Tiempo} = \frac{360°}{5.5°/\text{min}} = \frac{720}{11} \text{ minutos} \approx 65.45 \text{ minutos}$$
In 12 hours there are:
$$\frac{12 \times 60}{720/11} = \frac{720 \times 11}{720} = 11 \text{ superposiciones}$$
In 24 hours: $11 \times 2 = 22$ overlaps (not counting the one at 12:00 twice).
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