cars on the road
Quite an easy one.
If p is the probability of *not* observing a car in 5 minutes, then the probability of not observing a car in 20 minutes is p to the 4th power. So we know that:
p^4 = 1 - 609/625
It is then trivial to get the probability of observing a car in 5 minutes: 1-p = 3/5.
This assumes of course that "observing a car" means "observing at least one car" and not "observing exactly one car".
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