Suppose that an Enigma machine operator makes a mistake on one of every 80 letters transmitted on average independently of their mistakes on other letters. Today they have sent a 135-letter message. (a) Calculate the precise probability that the operator made a mistake on either 1 or 2 of the letters in this message. (b) Approximate the probability in (a) using the Poisson distribution. Do you think this approximation is a good one? Why?

A First Course in Probability (10th Edition)
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Chapter1: Combinatorial Analysis
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Suppose that an Enigma machine operator makes a mistake on one of every 80 letters transmitted on average independently of their mistakes on other letters. Today they have sent a 135-letter message.
(a) Calculate the precise probability that the operator made a mistake on either 1 or 2 of the letters in this message.
(b) Approximate the probability in (a) using the Poisson distribution. Do you think this approximation is a good one? Why?

Expert Solution
Step 1

Suppose that an Enigma machine operator makes a mistake on one of every 80 letters transmitted

Let p = probability of making mistake = 180

q = probability of not making mistake = 1 - p = 7980

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