Proability with Multiple Trials of Independent Events

I just looked at http://stattrek.com/online-calculator/binomial.aspx. That's not what you want. That's giving you the probability of x successes in n trials. You want to know the probability of n-1 failures followed by a single success.

If all you want is the CP for n trials, then that site's last number should give you that.

What are after? Do you want to know how long these challenges are likely to last, or that they'll end within n trials with say a 95% probability?

Thanks all for your responses. It was areal help.