Car washing

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

  1. Pim Registered Senior Member

    Messages:
    64
    Riddle thread

    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

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    Last edited: Jul 9, 2003
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  3. Reinstein Registered Senior Member

    Messages:
    45
    xy/(x+y)
     
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  5. Pim Registered Senior Member

    Messages:
    64
    we have a winner

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  7. Pim Registered 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?
     
  8. ryans Come to see me about a dog hey Registered Senior Member

    Messages:
    995
    01:05 pm
     
  9. Pim Registered Senior Member

    Messages:
    64
    That is not entirely correct. You can say it more precise.
     
  10. Reinstein Registered Senior Member

    Messages:
    45
    really? sounds right to me pim...
     
  11. Pim Registered 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. Reinstein Registered 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. Reinstein Registered Senior Member

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

    Messages:
    45
    wait thats not right, ok nevermind
     
  15. Pim Registered Senior Member

    Messages:
    64
    noon is 12 AM right?? The question was when do they coincide in the 12 hours after 12 AM
     
  16. Reinstein Registered Senior Member

    Messages:
    45
    no, noon is 12pm
     
  17. Pim Registered Senior Member

    Messages:
    64
    ah okay, sorry, we don't use this am-pm system here.
     
  18. Reinstein Registered 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. Pim Registered Senior Member

    Messages:
    64
    it is not that difficult

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    You can write the answer in a fraction, in stead of splitting it up in seconds...
     
  20. ryans Come to see me about a dog hey Registered 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!

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  21. Pim Registered 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. ryans Come to see me about a dog hey Registered 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. Pim Registered 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.
     

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