Sex determination in humans is a chance event. a family consisted of nine boys, including 2 sets of

Isn't it always a 50% chance lol

I believe you have to use the BINOMIAL THEOREM for this question.  

I am not sure if you've learned the binomial theorem before, but here's the general rule:

(a + b)^n = a^n + a^n-1(b) + a^n-2(b^2) + a^n-3(b^3) + ... b^n

Note: In each of these terms, there is usually a coefficient in front, except for the first and last terms (a^n and b^n).  You can find the coefficient of each term from Pascal's Triangle.

n = the size of the sample.  

There are one of two possible outcomes:

a = probability of being a boy
b = probability of being a girl  

The value of n is = 10 since there will be 9 boys and 1 girl.  

So, your binomial theorem for n = 10 would be:

(a + b)^10 = I am not going to expand the whole thing, but the term you would have to look for is 10(a^9)(b^1).  This is because there would be 10 children in total of which there will be 9 boys and 1 girl.  So, if you look at Pascal's Triangle, find the row with n = 10 and once you find that, you should be able to locate the term 10(a^9)(b^1).

So, for each child, the probability of being a girl is 0.5 and the probability of being a boy is 0.5: 0.5 + 0.5 = 1

So P(boy) = 0.5 & P(girl) = 0.5...So, a & b are = 0.5

So, your answer would then be 10(0.5)^9(0.5)^1.  The answer comes out to 0.009765625.  One of the choices given in your question is (1/2)^10 = 0.000976562.  There is an extra "0" in your choice.

So, I think the answer is D.

Hope that helps.  

Since they have already had 9 kids, no matter what sex the kids are, the 10th kid is 50% likely to be a girl and anotehr 50% to be a boy.
The "advanced" solution posted above is wrong.  If the question is:
A couple plans to have 10 kids.  What is the probability that all of them are boys?
Then Bio_World100's answer is correct...
But it's not.

(Use common sense here.  The past kids' genre doesn't affect the next one.)

Thanks for correcting me Thanos. 

To RADIOhead:

BTW, what is the correct answer to this question?  Just curious! 

So who, ultimately, is right?

We'll have to wait and find out! 

Sorry guys. I was never told the correct answer 

Thanks anyways

The answer has been verified as B - 50%.