If the probability of observing a car (read: at least one car) in 20 minutes on a highway is 609/625, what is the probability of observing a car (read: at least one) in 5 minutes (assuming constant default probability)?

Let p denote the probability of observing no cars in 5 minutes.

Then p^4 = 1 - 609/625 = 16/625

Or, p= 2/5

Consequently, the probability of observing at least one car in 5 minutes = 1-p = 3/5

*Edited on ***April 17, 2007, 11:35 am**