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June 21st, 2018, 06:02 PM  #11  
Math Team Joined: Oct 2011 From: Ottawa Ontario, Canada Posts: 13,125 Thanks: 914  Quote:
 the murderers prints are on the knife. Makes him ?% likely to be the killer  the murderer had blood on his clothes. Makes him ?% likely to be the killer Last edited by skipjack; June 21st, 2018 at 07:40 PM.  
June 21st, 2018, 06:12 PM  #12 
Member Joined: Jun 2018 From: UK Posts: 78 Thanks: 0 
The order does not matter the answer comes out the same: 90% + 10% x 70% = 97% chance guilty 
June 21st, 2018, 08:02 PM  #13 
Global Moderator Joined: Dec 2006 Posts: 19,528 Thanks: 1750 
Some of your terms are undefined. Also, you initially used the past tense, but then switched to using present tense for no reason. Can you make a simpler example based on rolling a die, say, and give the justification for each step?

June 22nd, 2018, 01:58 AM  #14 
Member Joined: Jun 2018 From: UK Posts: 78 Thanks: 0 
It’s hard to give a die example. To state the general method I’m using: Proposition A is x% likely because of fact 1 Proposition A is y% likely because of fact 2 How likely is proposition A taking into account fact 1 and fact 2? x% + (100%  x%) x y% So a simple example: Proposition A  ‘the dog is happy’. Fact 1  A dog psychologist tells you 60% dogs are happy Fact 2  The dog is wagging his tail (80% dogs happy if wagging tail) 60% + 40% x 80% = 92% 
June 22nd, 2018, 09:21 AM  #15 
Math Team Joined: Oct 2011 From: Ottawa Ontario, Canada Posts: 13,125 Thanks: 914 
There are 100 dogs. 60 (60%) are happy. If a dog wags tail: 80% chance it's happy. To be happy, must be a dog from the 60 that are happy. 60*.80 = 48 So if RinTinTin wags tail, there's a 48% chance he's happy. 
June 22nd, 2018, 09:37 AM  #16  
Member Joined: Jun 2018 From: UK Posts: 78 Thanks: 0  Quote:
The answer must be at least 80% because the dog is wagging his tail...  
June 22nd, 2018, 09:57 AM  #17 
Math Team Joined: Oct 2011 From: Ottawa Ontario, Canada Posts: 13,125 Thanks: 914 
See ya later buddy....this is kinda too confusing!! I don't have a damn dog anyway 
June 22nd, 2018, 11:34 AM  #18 
Member Joined: Jun 2018 From: UK Posts: 78 Thanks: 0 
OK I found the error I’m making. Thanks for all the help!

June 22nd, 2018, 12:32 PM  #19 
Math Team Joined: Oct 2011 From: Ottawa Ontario, Canada Posts: 13,125 Thanks: 914 
Please share it with us; may help another student...

June 24th, 2018, 01:14 PM  #20 
Member Joined: Jun 2018 From: UK Posts: 78 Thanks: 0 
Revised calculation below. Proposition 1  There was a creator god Big Bang is evidence for creator at 60% Fine tuning is evidence for the creator 75% probability so: 60% + 40% x 75% = 90% Prime mover is evidence for the creator 25% probability so: 90% + 10% x 25% = 92.5% chance of a creator god is To double check, I’ve done the inverse proposition below: Proposition 2  there was not a creator Big Bang is evidence against no creator 40% Fine tuning is evidence against no creator 25% probability so: 40% x 25% = 10% Prime mover is evidence against no creator 75% probability so: 10% x 75% = 7.5% chance of no creator god Last edited by skipjack; June 24th, 2018 at 01:54 PM. 

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