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April 13th, 2012, 01:09 PM  #1 
Member Joined: Nov 2011 Posts: 43 Thanks: 0  Probability Question
Need help D; Q: 30% of people at a baseball game bought cotton candy and water. And half of the people who bought water also bought cotton candy. Determine the probability of people who bought cotton candy. do you just take 0.3 and divide it by 0.5? D; and the answer's 0.6? someone tell me I'm right 
April 13th, 2012, 02:30 PM  #2  
Math Team Joined: Dec 2006 From: Lexington, MA Posts: 3,267 Thanks: 408  Re: Probability Question Hello, Chee! Quote:
There is insufficient information. Did everyone at the game buy either cotton candy or water (or both)? If so, we can solve the problem. Consider this Venn diagram. Code: * * * * * * * Candy * Water * * * * * * *30%* 30% * * * * * * * * * * * * * * Half of those who bought Water also bought Candy. Hence, the other half is: those who bought Water only (30%). Then 40% bought Candy only. Therefore: 70% bought Candy.  
April 13th, 2012, 02:58 PM  #3  
Member Joined: Nov 2011 Posts: 43 Thanks: 0  Re: Probability Question Quote:
 
April 16th, 2012, 11:45 PM  #4  
Senior Member Joined: Apr 2011 From: USA Posts: 782 Thanks: 1  Re: Probability Question Quote:
Great, that statement's true. And it gets you nowhere cause you already know the probability of W anyway, so there isn't anything to solve for. Now let's do it the way you seem to be attempting to do: The problem here is that you don't know WC. It's not .5, because that's CW. The problem is that you think .5 is WC when it's not. You don't know that number. It might help to understand what a conditional is, rather than trying to plug n chug numbers when you don't know which is which. By saying what's the probability of someone buying water given they have already bought candy, you are isolating down to only those people who bought candy. That is, you're isolating to the left circle in the Venn diagram. You toss out everyone else, so only the left circle exists now. So the probability of any of those people buying water is only the intersection people, out of the people who bought candy. To get the probability of the 30% in the intersection, out of the people in the left circle, you divide the intersection by the left circle. (Basic percentage rules, and where the equation comes from.) But you don't have a total for the left circle to divide by. .3 intersection divided by what? So you can't solve for that, in order to plug it in and then solve for the "what."  
April 16th, 2012, 11:46 PM  #5  
Senior Member Joined: Apr 2011 From: USA Posts: 782 Thanks: 1  Re: Probability Question Quote:
 

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