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# If the probability of observing a car in 30 minutes on a

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Senior Manager
Joined: 19 Apr 2008
Posts: 310
If the probability of observing a car in 30 minutes on a [#permalink]

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14 Sep 2008, 15:51
This topic is locked. If you want to discuss this question please re-post it in the respective forum.

If the probability of observing a car in 30 minutes on a highway is 0.95, what is the probability of observing a car in 10 minutes (assuming constant default probability)?
VP
Joined: 05 Jul 2008
Posts: 1373

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14 Sep 2008, 15:59
rpmodi wrote:
If the probability of observing a car in 30 minutes on a highway is 0.95, what is the probability of observing a car in 10 minutes (assuming constant default probability)?

I would say 1/3rd of 0.95

But who knows, This Q is pretty uncommon
Intern
Joined: 03 Sep 2008
Posts: 22

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14 Sep 2008, 16:29
You can't do this type of problem without knowing something about the probability distribution. Is it uniform, exponential, normal? This involves a continuous probability distribution and the GMAT deals primarily with discrete distributions.
Senior Manager
Joined: 19 Apr 2008
Posts: 310

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14 Sep 2008, 21:50
it says constant default probability , so that's the hint ...i don't know if this will be a GMAT problem but sure is an interesting question ...
SVP
Joined: 17 Jun 2008
Posts: 1504

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14 Sep 2008, 22:50
Not sure what the meaning of "default probability" is in "constant default probability". I give up. However, if asked in the exam, I will guess 1/3rd of 0.95.
Manager
Joined: 22 Jul 2008
Posts: 144

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15 Sep 2008, 16:02
Let prob. of seeing a car in 10 mins. be = p
Therefore, the prob. of not seeing the car is 1-p
Total 30 mins. prob = prob. at 10 mins + prob. at 20 mins + prob at 30 mins.
= p + (1-p)*p + (1-p)*(1-p)*p = 0.95
p[1 + (1-p) + (1-p)*2] = 0.95
This is a GP
p* [(1-p)^3-1]/(p-1) = 0.95
p^3 -3p^2 + 3p = 0.95
Hence p can be calculated from this cubic equation.
Re: interesting probability question   [#permalink] 15 Sep 2008, 16:02
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# If the probability of observing a car in 30 minutes on a

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