You went to bed at 9 o'clock and wound up your analog clock and set the alarm at 10 o'clock in the morning? How many hours could you sleep before the clock alarms?
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13 hours
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11 hours
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12 hours
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1 hour
The correct answer is D. 1 hour.
If you went to bed at 9 o'clock and set the alarm for 10 o'clock in the morning, then you would sleep for 9 - 10 = 1 hour.
Option A is incorrect because it would mean that you slept for 13 - 9 = 4 hours before the alarm went off. However, this is not possible because 4 hours is less than the 9 hours between 9 o'clock and 10 o'clock in the morning.
Option B is incorrect because it would mean that you slept for 11 - 9 = 2 hours before the alarm went off. However, this is not possible because 2 hours is less than the 9 hours between 9 o'clock and 10 o'clock in the morning.
Option C is incorrect because it would mean that you slept for 12 - 9 = 3 hours before the alarm went off. However, this is not possible because 3 hours is less than the 9 hours between 9 o'clock and 10 o'clock in the morning.
Therefore, the only possible answer is 1 hour.
This is the classic analog-clock riddle: a standard analog clock face only spans 12 hours and can't distinguish AM from PM. Going to bed at 9 (in the evening) and setting the alarm for 10, the clock mechanism only 'sees' the hour hand moving from the 9 position to the 10 position — a gap of just 1 hour on the dial — so the alarm rings after only 1 hour has passed on the clock face, even though in real elapsed time it would be many more hours.