
Probability and Statistics Basic Probability and Statistics Math Forum 
 LinkBack  Thread Tools  Display Modes 
May 24th, 2018, 01:24 PM  #1 
Newbie Joined: May 2018 From: Calgary, AB Posts: 5 Thanks: 0  Statistics Hi guys! Thank God I found this website, because I have definitely been struggling. I've been working on this problem for days and I still can't find the answer . I tried making a contingency table but I can't seem to find any values for when the roommate forgets to mail the letter intersected with either the letter being delivered or the letter not being delivered... You ask your roommate to mail a letter for you. Suppose there is a 0.22 probability your roommate will forget to mail it. Should your roommate mail your letter, there is a 0.91 probability that Canada Post will deliver the letter. If the letter is never delivered, what is the probability that your roommate mailed the letter? (Use four decimals in your answer) 
May 24th, 2018, 03:52 PM  #2 
Senior Member Joined: Sep 2015 From: USA Posts: 2,320 Thanks: 1232 
$P[\text{!mailed  !delivered}] = \dfrac{P[\text{!delivered  !mailed}]P[\text{!mailed}]}{P[\text{!delivered}]}$ $P[\text{!delivered  !mailed}] = 1$ $P[\text{!mailed}] = 10.22 = 0.78$ $P[\text{!delivered}] = (0.22)(1) + (0.78)(10.91) = 0.2902$ $P[\text{!mailed  !delivered}] = \dfrac{(1)(0.22)}{0.2902} = 0.758098 \Rightarrow 0.7581$ you get one w/o showing any work. show some work 
May 28th, 2018, 02:29 AM  #3 
Math Team Joined: Jan 2015 From: Alabama Posts: 3,261 Thanks: 896 
Imagine this scenario happening 10000 times. 22% of the time, 2200 times, your room mate forgets to mail it. The other 10000 2200= 7800 times, your room mate mails it. If your room mate does not mail it, Canada post cannot deliver it. The 7800 times your room mate does mail it, It is delivered 7800(0.91)= 7098 times. The other 7800 7098= 702 times, it is not delivered. Of the 2200+ 702= 2902 times the letter was NOT delivered, your room mate did, in fact, mail it 702 times. Given that the letter was not delivered, the probability that your room mate did, in fact, mail it was 702/2902= 0.2419.


Tags 
statistics 
Thread Tools  
Display Modes  

Similar Threads  
Thread  Thread Starter  Forum  Replies  Last Post 
Statistics  markosheehan  Probability and Statistics  3  September 10th, 2016 12:31 PM 
statistics help please  Lilian  Probability and Statistics  1  May 14th, 2014 08:59 PM 
statistics help  dumbledore  Advanced Statistics  0  November 30th, 2012 04:34 PM 
Statistics  cranddy  Probability and Statistics  33  August 21st, 2012 10:22 PM 
best introduction to statistics book? not applied statistics  ElMarsh  Advanced Statistics  2  October 23rd, 2009 06:10 PM 