Discussion
In a book of 520 pages, 390 typographical errors occur. Assuming Poisson probability law for the number of errors per page, what is the probability that a number sample of 5 pages will contain no error ?
Answer: D
The probability is \(e^{-3.75}\).
The mean number of errors per page is \(390/520 = 0.75\), so in 5 pages the mean is \(m = 5 \times 0.75 = 3.75\). For a Poisson variable, the chance of zero events is \(e^{-m}\):
\[ P(X = 0) = \frac{e^{-m} m^{0}}{0!} = e^{-3.75} \]
The options with positive exponents are greater than 1 and cannot be probabilities. \(e^{-4.75}\) adds 1 to the mean by mistake.
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