# Car washing

Discussion in 'Physics & Math' started by Pim, May 27, 2003.

1. ### PimRegistered Senior Member

Messages:
64

A nice puzzle for you:

Person A washes a car in x hours. Person B washes the same car in y hours. Question: how long will it take A and B together to wash this car?

It's not that difficult, let's see who wins

Last edited: Jul 9, 2003

Messages:
45
xy/(x+y)

5. ### PimRegistered Senior Member

Messages:
64
we have a winner

7. ### PimRegistered Senior Member

Messages:
64
Next riddle:

At noon, the long and short pointer on a clock coincide. When, during the next 12 hours, do they coincide again?

Messages:
995
01:05 pm

9. ### PimRegistered Senior Member

Messages:
64
That is not entirely correct. You can say it more precise.

10. ### ReinsteinRegistered Senior Member

Messages:
45
really? sounds right to me pim...

11. ### PimRegistered Senior Member

Messages:
64
If you would count on with 1:05:

1:05
2:10
3:15
4:20
5:25
6:30
7:35
8:40
9:45
10:50
11:55

This is not right. You know they don't coincide at 11:55. They don't coincide at EXACTLY 1:05:00 PM.. You can give an expression in parts of an hour if you know what i mean.

12. ### ReinsteinRegistered Senior Member

Messages:
45
ok i see what you mean. yeah it wouldn't be exactly 1:05 but i have to go to school,

laters

13. ### ReinsteinRegistered Senior Member

Messages:
45
ah! I was walking downstairs when i realized the answer. Is it 12am?

14. ### ReinsteinRegistered Senior Member

Messages:
45
wait thats not right, ok nevermind

15. ### PimRegistered Senior Member

Messages:
64
noon is 12 AM right?? The question was when do they coincide in the 12 hours after 12 AM

16. ### ReinsteinRegistered Senior Member

Messages:
45
no, noon is 12pm

17. ### PimRegistered Senior Member

Messages:
64
ah okay, sorry, we don't use this am-pm system here.

18. ### ReinsteinRegistered Senior Member

Messages:
45
nvm though, that answer was wrong, it has to be like 1.01 hours 6 minutes and some seconds but its way to early to do the math. have fun ryans

19. ### PimRegistered Senior Member

Messages:
64
it is not that difficult

You can write the answer in a fraction, in stead of splitting it up in seconds...

20. ### ryansCome to see me about a dog heyRegistered Senior Member

Messages:
995
The minute hand and the hour hand will coincide again at 13 hours and 5 mins if you take noon as being 12:00 hours

The hour, minute and second hand will all coincide at exactly
13 hours, 5 mins and 5 seconds or 1 hour, 5 mins and 5 seconds after noon!

21. ### PimRegistered Senior Member

Messages:
64
that's not correct ryans. I believe that there is no time that all 3 pointers coincide

edit: this is obviously wrong..

Last edited: Jul 14, 2003
22. ### ryansCome to see me about a dog heyRegistered Senior Member

Messages:
995
O.K. I am assuming that the steps of the minute hand are discrete and not continuous, i.e. the minute hand moves one unit clockwise each time the second hand goes past 12. Put up another problem, this one has too many different ways to percieve it. It is dependant on the clock you are using.

23. ### PimRegistered Senior Member

Messages:
64
I assumed the steps of the minute and hour pointer to be continuous. I should have said that, sorry. Anyway, here is the answer i think is right.

Between 12 pm and 12 am there are 11 coincidings. These are spaced equally from each other because the pointers have constant speed. So 11 coincidings in 12 hours means the first coinciding of the minute and hour hand will occur at 1 1/11 hour, which is 1 hour, 5 minutes and 27 seconds.