Probability of throwing a 6 in dice

[Dell]

i throw a dice until i get a result (6) 20 times, what is the probability that i will throw the dice more than 100 times?

i made X=amount of times i throw the dice

so now I am looking for P(X>100)

i think that i can say X~B(n, 1/6)
but the problem is that i don't know n, since it is anything over 100

 

[TaylorRatliff]

I don't think the Binomial distribution is appropriate here, check out the Negative Binomial Distribution: http://en.wikipedia.org/wiki/Negative_binomial_distribution

 

[jambaugh]

http://en.wikipedia.org/wiki/Geometric_distribution"

 

[Count Iblis]

In this problem you can always throw the dice 100 times. If you get 20 or more sixes, you needed to throw the dice 100 times or less to get exactly 20 sixes. If you haven't reached 20 yet, then that means that you would have reached 20 had you thrown the dice more than 100 times.

So, the desired probability is the probability that after 100 dice throws you have less than 20sixes.

 

[Dell]

thanks, that's great