Its been a while since I've posted on here - the dissertation is really dragging me down, plus I went away to snowdon for a weekend's climbing :D
Heres a quick one today, promise ill put a better one up next time :D
How can you put 10 lumps of sugar into three cups so there is an odd number of lumps in each cup?
Thursday, 17 February 2011
Tuesday, 1 February 2011
The Answer!!!
I'm sure a lot of you are going to be annoyed by this.... but it is just a simple case of where common sense is wrong :)
I have two children, and at least one of them is a son. What is the probability that the other is also a son?
If I have only one child, the probability is $\frac{1}{2}$ that it’s a boy.
But if I have two children, there are really four possibilities, depending on birth order: boy-boy, boy-girl, girl-boy, girl-girl. You know that at least one child is a boy, so you can exclude the girl-girl option. Of the three that remain, only one has two boys. So the probability that my other child is a boy is $\frac{1}{3}$.
Ta-dah!
I have two children, and at least one of them is a son. What is the probability that the other is also a son?
If I have only one child, the probability is $\frac{1}{2}$ that it’s a boy.
But if I have two children, there are really four possibilities, depending on birth order: boy-boy, boy-girl, girl-boy, girl-girl. You know that at least one child is a boy, so you can exclude the girl-girl option. Of the three that remain, only one has two boys. So the probability that my other child is a boy is $\frac{1}{3}$.
Ta-dah!
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