Originally Posted by
mr fantastic Pr(at least 30 days) = Pr(30 days) + Pr(31 days) + Pr(32 days) + ..... where:
Pr(30 days) = (0.999^30)(0.001) (why?)
Pr(31 days) = (0.999^31)(0.001) (why?)
etc.
You now have an infinite geometric series, and the formula for finding such a sum is well known.