It's likelihood per 150k flights per model (although likelihood per 150k flights per individual plane would give the same answer, it's just a weirder question).

It's likelihood per 150k flights per model (although likelihood per 150k flights per individual plane would give the same answer, it's just a weirder question). The assumption is that for the average plane all accidents happen independently from one another. Under such an assumption, the probability for any span of 150k flights is exactly the same as the probability for the first span of 150k. So it would not change the answer. If you roll a die a 1000 times, the odds of landing on a 6 on roll 999 is the same as it landing on a 6 the very first roll.