In the days before watches were invented, clocks were valuable items. There was a man who had one clock in his house. It kept good time, but one day he found that it had stopped. He had no idea what the correct time was. He walked to the next valley to visit his friend who had a clock showing the right time. He spent a little while chatting with his friend then he walked home. He did not know the exact length of the journey before he started. How did he manage to set his one clock correctly on his return?

Guest May 4, 2015

#6**+15 **

I am sorry Alan but I am still really struggling with this

I wind up my clock, and as I walk out my door the clock reads 12:00

I know it is slow so the real time is 12:00 + X minutes

I walk to my friends place I don't know how long I take but I will call it T minutes

When I arrive his clock reads 12:40

So

X+T=40 minutes

x=40 minutes - T

I chat for 10 minutes and then return home

I have been away from home for T+T+10 and my clock tells me that this adds up to 70 minutes

So

2T+10=70

2T=60

T=30 minutes.

I know that X=40-T so X=10. The clock is 10 minutes slow!

**WOW I worked it out - Now i just have to work out what you did in your answer**

Melody
May 6, 2015

#2**+10 **

His clock says t1 when he leaves. The time on his friends clock when he arrives is T, so his *apparent* journey time is δt = T - t1. He stays for a known time δT.

Assuming his apparent return journey time is also δt then his apparent round trip takes 2δt + δT or 2(T - t1) + δT. His clock now reads t2.

Assuming his clock runs at the right speed the interval Δt = t2-t1 is the true interval (though neither t1 nor t2 show the true time).

half the difference between the true interval and the apparent interval gives the required correction:

correction = (2(T-t1) + δT - Δt)/2

All the values on the RHS are known, so he knows the size of the correction. If it is positive the apparent interval is longer than the true interval, which means his clock is behind true time. If it is negative the apparent interval is shorter than the true interval and his clock is ahead of true time.

So he knows by how much and in what direction to adjust his clock.

.

Alan
May 5, 2015

#3**+10 **

I am having a problem getting my head around this Alan

Maybe you could do it with some pretend times

Melody
May 5, 2015

#4**+15 **

Example:

Time by his clock His friend's clock (true time)

Sets off **12.00** 12.10 (say)

Arrives at friend's place 12.30 **12.40**

( a 30min walk) Apparent journey time δt = 40 mins

Leaves friend's place 12.40 **12.50**

** **δT = 10 mins

Arrives home **13.10** 13.20

True journey time Δt = 13.10-12.00 (= 13.20 - 12.10) = 70 mins

Correction = (2*δt + δT - Δt)/2 = (2*40 + 10 - 70)/2 = 20/2 = 10 minutes

Correction is positive so he must add 10 minutes to the time his clock is showing.

.

Alan
May 5, 2015

#5**+10 **

**But his clock was stopped! **

Did he wind it up before he left to start it again! (no one told me that )

Melody
May 5, 2015

#6**+15 **

Best Answer

I am sorry Alan but I am still really struggling with this

I wind up my clock, and as I walk out my door the clock reads 12:00

I know it is slow so the real time is 12:00 + X minutes

I walk to my friends place I don't know how long I take but I will call it T minutes

When I arrive his clock reads 12:40

So

X+T=40 minutes

x=40 minutes - T

I chat for 10 minutes and then return home

I have been away from home for T+T+10 and my clock tells me that this adds up to 70 minutes

So

2T+10=70

2T=60

T=30 minutes.

I know that X=40-T so X=10. The clock is 10 minutes slow!

**WOW I worked it out - Now i just have to work out what you did in your answer**

Melody
May 6, 2015

#7

#8**+5 **

Thanks Alan.

That was a great question. It would have been better if I knew the clock was rewound before the man left but I guess the question did not say that it was not rewound :) I am not very good at thinking between the lines :))

Melody
May 6, 2015

#9**+5 **

I have added this one to the "**great answers to learn from**" sticky thread

(under puzzles)

Melody
May 6, 2015