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February 23rd, 2010, 10:36 AM   #1
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Probability Question that I can't figure out

Hello everyone

New here and doing some random probability questions in prep for the GRE. I found one and I know it is simple, but I am stumped, and there is no answer online. Here it is--any help is much appreciated.

Ralph is considering whether to use an umbrella. Previously, Ralph's friend Laura had used an umbrella 70% of the time. If Laura does not use an umbrella, the probability that Ralph will use an umbrella is .50. If Laura does use an umbrella, the probability that Ralph uses an umbrella is .25.

a) If Ralph uses an umbrella, what is the probability that Laura did not use an umbrella?
b)What is the probability that Ralph will use an umbrella?

Thanks so much in advance.
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February 23rd, 2010, 11:12 AM   #2
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Re: Probability Question that I can't figure out

There are four possibilities: both use, Lauren only, Ralph only, and neither. Lauren uses the umbrella 70% of the time, so B + L = 70% and R + N = 30%. R = 0.5 * (R + N), so R = N = 15%. Calculate B and L similarly, then add B + R to get the probability that Ralph uses an umbrella.
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February 23rd, 2010, 01:50 PM   #3
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Re: Probability Question that I can't figure out

Thank you very much for the response, but I am not quite following. What do you get as the actual probabilities to the questions under A and B?
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February 24th, 2010, 05:21 AM   #4
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Re: Probability Question that I can't figure out

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Originally Posted by indiegod
What do you get as the actual probabilities to the questions under A and B?
A hundred billion for A, and pi/4 for B.
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February 24th, 2010, 06:01 AM   #5
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Re: Probability Question that I can't figure out

That's pretty funny. Thanks for being so helpful.
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February 24th, 2010, 07:07 AM   #6
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Re: Probability Question that I can't figure out

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Originally Posted by indiegod
Thanks for being so helpful.
Thanks for your effort.
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