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Pascal grade 9 math contest question(can you ratio speed and time)

  1. Sep 11, 2014 #1
    I was given this question from the Pascal Math contest 2001:

    __________________________________________
    Cindy leaves school at the same time every day. If she cycles at 20 km/h, she arrives home at 4:30 in the afternoon. If she cycles at 10 km/h, she arrives home at 5:15 in the afternoon. At what speed, in km/h, must she cycle to arrive home at 5:00 in the afternoon?
    __________________________________________

    This was how I attempted to solve it:

    A difference in 10km/h (20km/h - 10km/h) results in her being 45 minutes late(5:15 - 4:30).
    So I ratio it:
    [tex] \frac{10km/h}{45min} = \frac{Xkm/h}{30min} [/tex]

    If a difference of 10km/h results in a 45 minutes delay then whats the speed difference(X) that results in a 30 minutes delay(5:00 - 4:30).

    I cross multiplied and got x = 6.6666....km/h.
    Subtracting that from 20km/h my answer was 13.3333....km/h

    The correct answer was 12 km/h.

    Now I have no idea how I got it wrong but I would assume that the mistake was the speed to time ratio. Is it correct to do that? Can you even ratio speed and time or is that incorrect and not allowed? If it's not possible then why?
     
  2. jcsd
  3. Sep 11, 2014 #2
    You need to find the total time it took her to get home, not just the additional time after the 20km/h trip.

    Edit: Here are some additional hints of how to set it up correctly.
    Distance = velocity x time.
    In this case, because Distance1 = Distance2 = Distance3 = Distance,
    Distance = Velocity1*Time1 = Velocity2*Time2 = Velocity3*Time3
    Time1: Unknown
    Time2: Time1 + 45 minutes
    Time3: Time1 + 30 minutes.

    You'll have to evaluate Time1 before you can solve this.
     
    Last edited: Sep 11, 2014
  4. Sep 11, 2014 #3
    I see. Thank you!
    So if total time was given that I could ratio?
     
  5. Sep 11, 2014 #4

    Ray Vickson

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    Set up two equations for T = starting time (hrs) and X = distance travelled (km). Remember,
    time travelled = distance/speed, so
    (1) 4.5 - T = X/20
    (2) 5.25 - T = X/10
    Solve these two equations to get T and X.

    Now you need to find v that gives 5-T = X/v, where T and X are known at this point.
     
  6. Sep 11, 2014 #5
    Sort of. Your ratio would be Velocity1/Time2 = Velocity2/Time1, but it's easier to leave it as Velocity1*Time1 = Velocity2*Time2.
     
  7. Sep 11, 2014 #6
    Yea I agree, I'll just solve it with your method then.
     
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