Assuming independent events (a bad assumption no doubt) then you're looking for 1 - the probability that for 20 years straight you don't have the event happenin

Assuming independent events (a bad assumption no doubt) then you're looking for 1 - the probability that for 20 years straight you don't have the event happening. - E event happens in given year - N event doesn't happen in given year Then P(E) = 1/125. Conversely P(N) = 1-1/125. We're looking for 1-P(NNNNNNNNNNNNNNNNNNNN) = 1 - (1-1/125)^20 = 15% Though I'm sure someone will tell me how I'm wrong