A man living in the 19th Century wakes up one morning to find that his wind-up clock has stopped. It is the only clock he has.
He is able to determine the time by using the following procedure:
He walks to a friend’s house. Since he’s never walked to his friend’s house before, nor does he know the distance, he cannot make an accurate estimate of how much time has gone by during his journey. His friend has a functioning and accurate grandfather clock. After visiting the friend for some time, he returns home and is able to determine what time it is in order to set his clock correctly.
He doesn’t know his walking speed as measured in m.p.h. However, he always walks at the same speed, and is aware of this. The travel time to his friend’s house is the same as his return.
How was the man able to set his clock correctly when he got home?