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ON a city street, car backfires are heard 8 times per hour. Use the poisson distribution to find an exact expression for the prob. that a car backfire is heard at most once in a given hour. Do not simplify or evaluate your answer.

Now from my understanding, in the poisson distribution, mean = variance = 'parameter'

In the question we can assume that the mean equals 8, thus letting the parameter = 8.

A car backfire is heard at most once in a given hour.

= Probability of hearing 0 backfires + probability of hearing 1 backfire

= 8^0 e^(-8) / 0! + 8^1 e^(-8) / 1!

= e^(-8) + 8e^(-8)

= 9e^(-8)

Is this the correct solution? Thanks for your time