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  #1  
Old 10-19-2002, 04:03 PM
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Default Typo in Sklansky Column

Probably several readers have already pointed out this typo, but Sklansky's column in the October 11, 2002 issue of Card Player contains the following: "There are 30 children in your child's class. How many are either boys or redheads? If I tell you that there are 13 girls and four redheads, that doesn't mean the answer is 17...To get the right answer, you would in fact have to subtract the number of redheaded boys from 17." Clearly, the "13 girls" should be "13 boys." Obvious typos such as this one will not trouble the reader with a smattering of knowledge about probability. The totally unsophisticated reader, asked the question "See Why?", will drive himself crazy in a doomed attempt to see why. A published correction would be useful.
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  #2  
Old 10-19-2002, 05:07 PM
Mike Haven Mike Haven is offline
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Default Re: Typo in Sklansky Column

no

there are 17 boys (13 girls)

there may be, say, 2 redheaded boys, so of the 4 redheads 2 are girls

so there are 19 boys or redheads (in this case)

or, take 2 redheaded boys from 17 boys = 15, and add the 4 redheads = 19 boys or redheads

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  #3  
Old 10-19-2002, 09:18 PM
ronniejames ronniejames is offline
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Default Re: Typo in Sklansky Column

You need to read more carefully. You'll notice that you consider that the answer might be 19. Sklansky writes: "To get the right answer, you would in fact have to subtract the number of redheaded boys from 17. See why?" There are two possible typos in the column. If the published "13 girls" was intended to be '13 boys', then everything else is correct. If, as you suggest, we are to subtract the 13 girls from a total of 30 children, giving us 17 boys, then the two places where the number 17 appears should read, 21. I do not, even for a moment, entertain the possibility that Sklansky gave the wrong answer. It's too simple and he's far too competent.
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  #4  
Old 10-20-2002, 06:55 PM
Mike Haven Mike Haven is offline
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Default Re: Typo in Sklansky Column

i didn't see his article

i gave an example to show that his instructions were correct

his instructions are correct:

"There are 30 children in your child's class. How many are either boys or redheads? If I tell you that there are 13 girls and four redheads, that doesn't mean the answer is 17...To get the right answer, you would in fact have to subtract the number of redheaded boys from 17."

if i tell you there are 13 girls in a class of 30 there are 17 boys

if i tell you there are 4 redheads they may be boys or girls

if there are, say, two redheaded boys then you have to subtract this number from the 17 boys = 15

then add the 4 redheads = 15 + 4 = 19

otherwise you are double-counting the redheaded boys


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  #5  
Old 10-21-2002, 03:29 AM
ronniejames ronniejames is offline
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Default Re: Typo in Sklansky Column

Mike, read Sklansky's words: "To get the right answer, you would in fact have to subtract the number of redheaded boys from 17." Now, you can't subtract any number of redheads from 17 that will yield an answer of 19! If we agree that there are 17 boys and 4 redheads, then the number of children who are either boys or redheads can be 21 (if all the redheads are girls), 20, 19, 18, or 17 (if all the redheads are boys). So, if Sklansky wants us to subtract the number of redheads from 17, something is obviously wrong: he should be asking us to subtract from 21. I suggest that he intended to write, 13 BOYS and 4 redheads, and somehow it came out in print as 13 GIRLS.
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  #6  
Old 10-21-2002, 04:09 AM
Mike Haven Mike Haven is offline
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Default Re: Typo in Sklansky Column

"To GET the right answer ..."

the calculations don't STOP at "... you would in fact have to subtract the number of redheaded boys from 17"

they INCLUDE that process

to make your ham and eggs you have to get a chicken first - well - maybe not - but you take the point?
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  #7  
Old 10-21-2002, 04:17 AM
ronniejames ronniejames is offline
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Default Re: Typo in Sklansky Column

I'm afraid I have to throw in the towel. I just don't understand what you're driving at. It really isn't all that complicated. Really.
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  #8  
Old 10-21-2002, 04:38 AM
BruceZ BruceZ is offline
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Default Re: Typo in Sklansky Column

There are 30 children in your child's class. How many are either boys or redheads? If I tell you that there are 13 girls and four redheads, that doesn't mean the answer is 17.

No, why would anyone think it is 17? One might naively think it is 21, since this is 17 boys + 4 readheads. But then you would have to subtract the redheaded boys from 21 not 17. Clearly this is a typo and both bold words above should be boys. Then you subtract redheaded boys from 17 to get the final answer. This is how the demonstration of the inclusion-exclusion principle should be done.

boys OR redheads = boys + readheads - redheaded boys
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  #9  
Old 10-21-2002, 09:08 AM
Mike Haven Mike Haven is offline
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Default Re: Typo in Sklansky Column

yep - it's a typo alright - i think it should be corrected to save anyone becoming confused
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  #10  
Old 10-21-2002, 06:32 PM
happyjaypee happyjaypee is offline
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Default Re: Typo in Sklansky Column

boys OR redheads = boys + readheads - redheaded boys

Skanlsky explanation would work whit the fellowing equation too:

boys OR redheads = <font color="red"> boys - redheaded boys </font color> + readheads

Is thought process is just in the midlle of the explanation. I agree tought that it is confusing. I reed the post on CP and got a little lost. I Understanded it reading your posts
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