# Probability question - Binomial distribution

1. Sep 1, 2010

### thereddevils

1. The problem statement, all variables and given/known data

A game is played by tossing two unbiased coins repeatedly until two heads are obtained in the same throw. The random variable X denotes the number of throws required. Find the expression for P(X=r).

2. Relevant equations

3. The attempt at a solution

It looks to be a binomial distribution but the number of trials could be infinity. I have no idea to class this into which distribution(so far i have learned binomial, poisson and normal).

The best i can get is

P(X=r)=nCr (1/4)^r (3/4)^(n-r)

2. Sep 1, 2010

### Office_Shredder

Staff Emeritus
Re: probability

What is n supposed to be?

The number if trials could be infinite, but the probability of that happening is zero. The setup is similar to if someone just flipped one coin until they got a heads; do you know how to solve that problem?

3. Sep 2, 2010

### thereddevils

Re: probability

It's an infinite geometric series probability(not sure what to call that).

1/2 + (1/2)^2 + (1/2)^3 + (1/2)^4 + ...

but now two coins are flipped together, i am not sure how to do that here.