
Probability and Statistics Basic Probability and Statistics Math Forum 
 LinkBack  Thread Tools  Display Modes 
October 19th, 2016, 06:38 AM  #1 
Newbie Joined: Oct 2016 From: United States Posts: 1 Thanks: 0  Probability/Combinations confusion
Hi everyone, I'm having some difficulty understanding when to use combinations and when to just multiply specific probabilities together for a given outcome... So we have 10 marbles, 7 red, 3 blue. How do I find the probability of getting four red marbles in a row? I tried two methods and they both seem correct to me, but they give me different results and I'm not sure why. Probability of desired event = Number of desired events/ number of total events (7/10) * (6/9) * (5/8 ) * (4/7) = 840/5040 = .166 or (number of ways to choose 4 red marbles) = 1 (number of total ways to chose 4 marbles out of 10) = (10 choose 4) = (10*9*8*7)/(4*3*2*1) = 5040/24 = 210 1/210 = .0049 I'm not sure which is correct, and why the two methods give me different answers. Please Help! Last edited by skipjack; August 7th, 2017 at 09:49 PM. 
August 7th, 2017, 09:52 PM  #2 
Global Moderator Joined: Dec 2006 Posts: 18,250 Thanks: 1439 
The first is correct.


Tags 
combinations, confusion, probability 
Thread Tools  
Display Modes  

Similar Threads  
Thread  Thread Starter  Forum  Replies  Last Post 
Combinations and probability  CUPOFCOFFEE  Probability and Statistics  3  May 13th, 2016 08:25 PM 
Probability with Combinations  jbergin  Advanced Statistics  1  December 30th, 2014 02:38 PM 
Probability with Combinations  jbergin  Probability and Statistics  7  December 26th, 2014 08:45 AM 
Probability ofa ll combinations of traits  caters  Probability and Statistics  0  April 29th, 2014 12:07 PM 
probability and combinations  Jarwulf  Probability and Statistics  3  November 19th, 2012 10:21 PM 