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The Gap is greater than 8 seconds, it means every next vehicle will arrive at least after 8 seconds.
We have to find the probability that no vehicle arrives in the gap of 8 seconds.

Expected vehicles per hour = 900/hour

λ = expected vehicles per 8 seconds = (900*8)/(60*60) = 2

λ = 2

P(X=x) = $(e^{-λ} *λ^x)$/x!

P(X=0) = $e^{-2}*2^0$/0! = e^-2 = 1/e^2 will be the answer.