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The Gardner Sisters I

Probability puzzles require you to weigh all the possibilities and pick the most likely outcome.


Puzzle ID:#32683
Fun:*** (2.71)
Difficulty:** (1.76)
Submitted By:tsimkinAus*****!




Gretchen and Henry were discussing their new neighbors, the Gardners. Gretchen mentioned that she met two of the daughters, and they each had blond hair.

"I have met all of the sisters," replied Henry, "and the probability that both of the girls you met would have had blond hair, assuming you were equally likely to meet any of the sisters, is exactly 50%. Do you know how many children there are?"

After thinking for a minute, Gretchen asks if the family is abnormally large. When Henry replies that it is not, Gretchen tells him how many girls are in the family. What number did she say?

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Aug 28, 2006

great how u worked it out to fit wow!!!!!!!!
Sep 01, 2006

ya i agree with Mattroxs
Dec 29, 2006

This is wrong- you are expressing a certainy as a probability of less than 1. If there are four girls, three of whom are blond and you meet two of them, then the probability of one of them being blond is 100%. There simply aren't enough non-blonds to pick.
The correct answer is three daughters- two blond and one non-blond. The probability of picking two blonds is the same as picking one of each ie 50%.
Dec 29, 2006

harryharry -- The problem states that the probability that they are BOTH blond is 50%. I agree that the probability that AT LEAST ONE is blond is 100%, but that is irrelevant.
Dec 30, 2006

Yes you're quite right- I understand it now. Very interesting. Thanks
Mar 04, 2007

i did not get it wat so eva to bad 4 me!!!!!!!!!!!
Apr 14, 2007

yeah thats a really good one
Apr 29, 2007

Let there be n sisters of whom m are blond. Meeting two, the probability of both blond is then m/n * (m-1)/(n-1) = 1/2. This leads to the equation m^2 - m = (n^2-n)/2. Now for any given n, can we find integer solutions for m. From the puzzle, n is at least 3.
n=3: m^2 - m - 3 = 0. No integer solutions.
n=4: m^2 - m - 6 = 0. Yes m = 3.
n=5: m^2 - m - 10 = 0. No integer solutions.
n=6: m^2 - m - 15 = 0. No integer solutions.
etc. We are basically looking for a triangular number (3,6,10,15 etc) which has factors that differ by one. We have found 2*3=6 so 3 is a solution.
The triangular numbers continue..(21, 28, 36, 45, 55, 66, 78, 91, 105, 120, ...) At this point we can probably stop looking because we are up to the 15th triangular number without finding another solution and the family was not inordinately large.
Loved it!!
Jul 16, 2007

I didn't use the logical answer. I just guessed!
Dec 09, 2008

Nice puzzle. Jimbo, I like your analysis of the solution space.
Jan 06, 2010

I reached the conclusion that if they had a total number of daughters equal to n, from whom b are blond. Then the following equation holds true:
(b/n)*((b-1)/(n-1)) = 0.5.
Then I stopped and checked the answer
Sep 16, 2010

Let there be n girls and b of them blonde. The total possible ways to meet 2 girls is n choose 2. The total possible ways to meet 2 blonde girls is b choose 2. List a few k choose 2 integers for k = 2, 3, 4, ... and you get 1, 3, 6, ...

As soon as you see 3/6, you can see that b=2 and n=3.
Sep 16, 2010

Sorry for typo. I meant b = 3 and n = 4.
Jun 28, 2011

This is WRONG! Gretchen told him how many girls ARE IN THE FAMILY not how many sisters there are. Gretchen did not answer the question.

Also even if there really are 4 girls in the children then that means that there are at least 5 girls in the family including the MOTHER! 4 girls, half of which she saw( the two blonds) and then the mother!
Jun 28, 2011

Bigbanggoo -- thank you for your constructive help on this problem. I should be clear -- I was using the generally accepted form of "girl" meaning "female child, from birth through adolescence". It is true that with the wider construct of "girl" meaning "any female", you should include the mother. It is also true that this family may have had pets, who themselves may have been female, and therefore should also be included in the number of "girls" Gretchen mentions at the end. I am appropriately chastened by your insight. Thank you for taking the time to add value to this question and this site.

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