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Post by alanseago on May 28, 2010 14:22:12 GMT
I just remembered a version of this from my younger days (?) If you toss a coin, the possibility of it coming down heads is 50%. If you toss seven times and it comes down heads, you toss again what is the possibility of it coming down heads? 50%
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Post by housesparrow on May 28, 2010 15:42:26 GMT
No, Rob is right - I've just worked it out on paper. Unfortunately I can't tabulate it here so I'll try to rearrange thus:
Ist child = Boy - 2nd child boy (50%) or girl (50%) 1st child = Girl - 2nd child = girl(50% chance) or boy
So the chances of having one of each are the same as those of having two of the same sex. So if you eliminate the two boy families, you are left with double the chance that the other child will be a boy (I think)
The question cleverly doesn't say that the parents are expecting their second child; the girl could be the first or the second child.
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Post by motorist on May 28, 2010 15:51:55 GMT
Oi. Page. Stop trying to sneak maths onto the Stub. Everyone, it's maths! He is trying to make us do maths! About time you learned some, you can't always make do with just 8 fingers and 2 thumbs for counting
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Post by everso on May 28, 2010 17:47:02 GMT
Q/ I have 4 children, 2 girls, 2 boys. None of whom were born on a Wednesday. Calculate the probability of a nervous breakdown.
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Post by everso on May 28, 2010 17:51:46 GMT
I'm with Trubbs. My brain hurts. I'm very good at arranging flowers though.
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Post by bonbonlarue on May 28, 2010 20:11:43 GMT
I'm with Trubbs. My brain hurts. I'm very good at arranging flowers though. I'm shit at flowers....only fit for breeding it seems....
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Post by motorist on May 28, 2010 20:15:55 GMT
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Post by rjpageuk on May 28, 2010 22:22:27 GMT
This is surely a phallacious syllogism. You are correlating the percentage of male and female in the population with the likelihood of a mother having a male or female baby. Hey you made me learn a new logical fallacy, thanks for that, but this is not one of them. I am not correlating anything with anything, the prior conditions for the problem were set as part of the problem (two children families, one of which is a girl). Humans have problems understanding conditional probability, and this (and Monty Hall, and various others are all examples of this, I was hoping to add some of them to this thread). Is anyone going to have a go at the second question? I think I am wasted on you lot
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Post by rjpageuk on May 28, 2010 22:24:27 GMT
I just remembered a version of this from my younger days (?) If you toss a coin, the possibility of it coming down heads is 50%. If you toss seven times and it comes down heads, you toss again what is the possibility of it coming down heads? 50% Yeah but if you have the prior information that someone has tossed a coin twice, and that at least one of the coin tosses is a head then there is a 1/3rd chance that they have tossed two heads .
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Post by alanseago on May 29, 2010 12:01:22 GMT
But after each toss, you have full information of the result. Rather like having babies, once they are born you know their gender. What are the odds on the next one? I should say 'going forward' somewhere around here.
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Post by housesparrow on May 29, 2010 20:29:28 GMT
Alan - as I said earlier, we are not trying to work out the likely sex of the second, because two children have been born.
rob - As for the "born on a Wednesday" - I think we have all dismissed this as a red herring. But I await further enlightenment.
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Post by rjpageuk on May 29, 2010 22:31:16 GMT
Alan - as I said earlier, we are not trying to work out the likely sex of the second, because two children have been born. Indeed! I am very glad that at least you and alan are playing along (humouring maybe?) me ;D rob - As for the "born on a Wednesday" - I think we have all dismissed this as a red herring. But I await further enlightenment. It isnt a red herring, it matters. Try to extrapolate what we did in the first question but using the information of what day the child is born on.
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Post by housesparrow on May 29, 2010 22:36:45 GMT
I've tried, but my brain keeps coming back to...
...."what the f*** difference does the day of birth matter?"
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Post by trubble on May 29, 2010 22:40:25 GMT
I've tried, but my brain keeps coming back to... ...."what the f*** difference does the day of birth matter?" It doesn't. Both are irritating but you don't have the info about them already having a daughter in Q 2 so - less useful info, really, and a tad misleading. Q 1 is like reading the Telegraph and Q 2 more like the Daily Mail.
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Post by rjpageuk on May 29, 2010 23:05:37 GMT
I've tried, but my brain keeps coming back to... ...."what the f*** difference does the day of birth matter?" Ok I will put you out of your misery. I think the best way to see the answer to the problems is to see all of the possible combinations, so I have drawn them in a grid. Question 1: Question 2:
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Post by housesparrow on May 30, 2010 6:02:40 GMT
Well, the first table is the one I used at home, but didn't know how to put here (see my reply 21)
As for the second.....no wonder I can't follow knitting patterns.
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Post by Weyland on May 30, 2010 7:39:37 GMT
I've tried, but my brain keeps coming back to... ...."what the f*** difference does the day of birth matter?" Ok I will put you out of your misery. I think the best way to see the answer to the problems is to see all of the possible combinations, so I have drawn them in a grid. Question 1: Question 2: I'm a frayed knot, rjp. The only possible outcomes are BG and GG, so the answer is 50%. For your answer there'd have to be three possible outcomes. QED. Can't get my head around the other bit, on account of I've got to rush off to the airport with a swollen ankle.
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Post by housesparrow on May 30, 2010 8:02:45 GMT
The first table is a bit misleading because of the way the grey boxes are labelled as "fails", but yu have to include the BG and GB in the calculation.
But the second table is just crazy. Again, we don't know whether the girl-born-on-a-Wednesday is in the first or second child, so we can't rule out the possibility that the second child is a girl born on any day of the week - including Wednesday! Therefore the table should look just like the one at the top.
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Post by alanseago on May 30, 2010 9:03:40 GMT
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Post by Patrick on May 30, 2010 12:18:00 GMT
Love it!
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