# Homework Help: Probability - conditional

1. Feb 5, 2009

### mind0nmath

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

boy and girl independently flip each a biased coin with probability of heads p1 for boy, and p2 for girl. they record the number of flips needed until heads shows up.

what is the probability that they tie?

2. Relevant equations

I just know that we should define a random variable to condition on.

3. The attempt at a solution

LOST!!

2. Feb 6, 2009

### nitro

First of all, the question is not clear.
"they record the number of flips needed until heads show up"
do they stop recording when the first head appears?

In any case, here is hint..
why don't you start with the same probabilities for the boy and the girl, maybe with some number like (p=0.5) . Then find out the expected number of flips until a head shows up.. At this point, you'll be able to compute the desired probability.

Go back the question and plug the p_1, p_2 to get your answer

3. Feb 6, 2009