This was a neat one: thanks to the friend who passed it on to me. One's first thought is to go at it with a breaking-rocks approach, piling up chains of probabilities.
If you stand back from it for a few minutes though & meditate on the immensity of the universe and the insignificance of a human life, you see it's really simple.
That's the good math. The bad math was in last weekend's Radio Derb:
We're coming up to the three hundredth anniversary of the Defenestration of Prague, generally taken as the start point of the Thirty Years War …Among the annoyingly numerous reader responses to that, my favorite was this one:
Mr Derbyshire: I too would rather be living in 1918, but I think you will find we are nearing the four hundredth anniversary of the Defenestration of Prague.