# Not so simple coin experiment

• Mar 1st 2010, 10:48 AM
anzej
Not so simple coin experiment
Can someone help me with some reference or any kind of a hint for the problem, which goes like this:

You flip each of n coins repeatedly until it shows heads. Let X be the number of coins that require at least five flips. Find P(X = 0) and E[X].

I have tried to approach it by employing the geometric distribution for random variables (Olofsson: 118) but I quess this is not the right way to do it, since that could be appropriate if there was only one coin and it would be flipped for X times until the head "occurs".
• Mar 23rd 2010, 08:21 PM
mr fantastic
Quote:

Originally Posted by anzej
Can someone help me with some reference or any kind of a hint for the problem, which goes like this:

You flip each of n coins repeatedly until it shows heads. Let X be the number of coins that require at least five flips. Find P(X = 0) and E[X].

I have tried to approach it by employing the geometric distribution for random variables (Olofsson: 118) but I quess this is not the right way to do it, since that could be appropriate if there was only one coin and it would be flipped for X times until the head "occurs".

Calculate p = Pr(a coin requires at least five flips until showing heads).

Then X ~ Binomial(n, p).