Does he have a brother or sister 回答

When you flip a coin, it does not matter what you get the first time.
The probability to get Head or Tail second time is independent on the result of the previous flip.

So the answer of the problem posted on August 19 is 50%.

This was easy. Most of you got the correct answer I am sure.

Now the other problem, posted on August 16 is a bit counter-intuitive.

Mr. Smith has 2 children and he was walking down the street with his boy.
That means among all combination of 2 children, Boy+Boy, Boy-Girl, Girl-Boy, Girl-Girl, the option of having two girls is out. Right?

This boy is twice as likely to have a sister than a brother.

Did you get it right?
Interesting, huh?
I hope you had fun.

The Japanese version of this page is provided on August 13.