Math Challenge #3.1 - Solution posted in thread

[kranky]

You are in a chat room one night and start talking to a girl you have never met. She tells you she has one sibling. What are the odds that the other child in her family is a boy?

 

[Wangel]

1 in 3

 

[tom3]

any explainations??

 

[Czar]

1/2

 

[G41184b]

2/3
Punnett squares in biology say 2 out of four are girls and 2 out of 4 are boys. 1 is alreasy a girl so that leaves 3 left, 2 being a boy and one being a girl

Punnett square [possibly chromosome combinations]
XX XY
XX YX

I am probably way off, been years since biology class, and even then I didnt really pay attention/

 

[allan120]

1:2. Each new child is an independent event. This situation is akin to flipping a coin twice.

The Punnett square only applies to the probability of the sex chromosome makeup of each offspring. Whether your conversational partner is male or female is irrelevant.

Of course we're leaving out hermaphrodites, tho I guess we can make the argument for Y chromosome == boy.

Edit: Also forgot the probability that she is an identical twin, which would mean it was just one event, not two. Ugh, biological math questions are messy.

 

[robp]

Option:


Girl than Boy
Girl than Girl
Boy than Girl
Boy than Boy



1/4 chance.
or use powerset, 2^2.


Of course, I am probably wrong.

 

[Chooie]

the other thread is getting long in tooth... let's do this one

 

[thebestMAX]

Come on, Man,

Just like all the other questions like this. 50/50. Either its a boy or it isnt! BTW, since Sex is determined by the father if you look up the statistics for the particular period of time her sibling was born, you might get an edge. (More boys born during periods of war than peace on so on.)



No offense meant.

 

[thebestMAX]

Do more boys or girls use chatrooms?

 

[BA]

0. Any girl you have never met is either a. and old man pretending to be a girl, or b. a cop pretending to be a girl. In either case, there probably aren't any children in the family

 

[denali]

BA, is correct!

 

[thebestMAX]

The odds all say that her sibling is BA or that she is !

 

[thebestMAX]

Hey, BA

Just gave you a 9 (10 really but I cant admit to it).

Lawrence, Ks is one of the biggest college party towns in the US. Used to live in KC and the University of Kansas was one of my big accounts. Lots of good memories there.

 

[BA]

What are you doing talking to my sister? She's 13! And yes it is quite fun, I just had to go home and get my toga last weekend...

 

[piku]

What are the chances that the girl you are talking to is a girl?

 

[Imported]

Slim to none?

 

[AudioBitch]

ok folks

there are only 2 siblings...so that means denominator has to be 2...and we already kno one is a girl...so numerator equals 1

we get...1/2


congrats folks

hahahhaa

 

[kranky]

Sorry I couldn't check back sooner. I want to clarify that there is no trickiness here. No identical twins or chromosome weirdness.

For those of you who followed the other thread (Math Challenge No. 3), it might be obvious that the answer is NOT going to be 50/50!

 

[kranky]

It's not 1/2. Anyone else want to take a shot?

 

[Demon-Xanth]

42

That's always the answer.

 

[VoodooExtreme]

Try switching!!! That always increase the odds

 

[chess9]

1 in 3.

 

[kranky]

Congrats to G41184b with the correct answer: 2/3.

There are four possible combinations with two children (B=boy, G=girl) : BG, GB, GG, BB.

We know there is one girl for sure, so that eliminates BB, leaving BG, GB, GG. Of those three, two have a boy sibling and one has a girl sibling, meaning a 2/3 chance of the other child being a boy.

 

[HoopDogg]

has nothng to do with biology/physics

it's simple probability

50/50 my friend. Unless you're talking to some gal from the amazon island, where they're all women (or the ones who use the pc's anyways...)