
Advanced Statistics Advanced Probability and Statistics Math Forum 
 LinkBack  Thread Tools  Display Modes 
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 isany 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  
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 Quote:
 
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  
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 Quote:
 

Tags 
figure, probability, question 
Thread Tools  
Display Modes  

Similar Threads  
Thread  Thread Starter  Forum  Replies  Last Post 
Please help, I cannot figure this out, very short question.  Nmbvc0  Algebra  7  March 6th, 2013 07:49 AM 
Question in centroid for given figure  rsoy  Physics  4  November 10th, 2011 05:15 AM 
Probability Question that I can't figure out  indiegod  Probability and Statistics  1  February 23rd, 2010 03:11 PM 
Polynomial Question  Nobody Can Figure it Out  Chronic12  Algebra  1  October 22nd, 2008 12:01 AM 
Please Help, I cannot figure this out, very short question.  Nmbvc0  Calculus  0  December 31st, 1969 04:00 PM 