# Math Help - probability of at least two.

1. ## probability of at least two.

Hi;
I'm having trouble with this question.

Three coins are tossed what is the probability of getting three
heads if it is known that at least two heads were obtained?

Thanks.

2. ## Re: probability of at least two.

Originally Posted by anthonye
three coins are tossed what is the probability of getting three
heads if it is known that at least two heads were obtained?
$\\thh\\hth\\hht\\hhh$

3. ## Re: probability of at least two.

So the answer is 1/2. I just want confirmation because my book says 1/4.

4. ## Re: probability of at least two.

OH also the formula is it P(A given B) = (A and B)/P(B) I sure this is correct but whats P(A and B) in this question.

Thanks.

5. ## Re: probability of at least two.

Originally Posted by anthonye
So the answer is 1/2.
NO! Plato showed four ways you could have "at least two heads" and only one of them has three heads. The "probability of three heads" given that "you have at least two heads" is 1/4.

I just want confirmation because my book says 1/4.

6. ## Re: probability of at least two.

Ok thank you I went and looked at it again I see now.

Thank you both.