Re Genghis' statistics, it depends whether the risk of an accident in any given year is independant of the probability in any other year.
If independant, the probability of NOT having an accident is (for 1 in 400)399 in 400, so the chance of not having an accident in 20 years is 99.75 raised to the 20th power (or about 95%). For 1 in 2000, close to 99%.
I suspect Genghis is a good enough engineer to actually know that, and was simplifying the argument for pilots to understand, but that's another discussion..........
Dangerous stuff statistics - worse than flying.