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## Homework Statement

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).

## Homework Equations

## 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)