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 February 23rd, 2010, 10:36 AM #1 Newbie   Joined: Feb 2010 Posts: 4 Thanks: 0 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. February 23rd, 2010, 11:12 AM #2 Global Moderator   Joined: Nov 2006 From: UTC -5 Posts: 16,046 Thanks: 938 Math Focus: Number theory, computational mathematics, combinatorics, FOM, symbolic logic, TCS, algorithms 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. February 23rd, 2010, 01:50 PM #3 Newbie   Joined: Feb 2010 Posts: 4 Thanks: 0 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? February 24th, 2010, 05:21 AM   #4
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Re: Probability Question that I can't figure out

Quote:
 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. February 24th, 2010, 06:01 AM #5 Newbie   Joined: Feb 2010 Posts: 4 Thanks: 0 Re: Probability Question that I can't figure out That's pretty funny. Thanks for being so helpful. February 24th, 2010, 07:07 AM   #6
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Re: Probability Question that I can't figure out

Quote:
 Originally Posted by indiegod Thanks for being so helpful.
Thanks for your effort. Tags figure, probability, question Thread Tools Show Printable Version Email this Page Display Modes Linear Mode Switch to Hybrid Mode Switch to Threaded Mode Similar Threads Thread Thread Starter Forum Replies Last Post Nmbvc0 Algebra 7 March 6th, 2013 07:49 AM r-soy Physics 4 November 10th, 2011 05:15 AM indiegod Probability and Statistics 1 February 23rd, 2010 03:11 PM Chronic12 Algebra 1 October 22nd, 2008 12:01 AM Nmbvc0 Calculus 0 December 31st, 1969 04:00 PM

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