We know that certain bacteria are present in water in a proportion of 3 bacteria per cm3 . What is the probability that a sample of 10 cm3 contains 10 bacteria?

A First Course in Probability (10th Edition)
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Chapter1: Combinatorial Analysis
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We know that certain bacteria are present in water
in a proportion of 3 bacteria per cm3
. What is the probability that a sample of 10 cm3
contains 10 bacteria?

Expert Solution
Step 1: Determine the given variable

Given:
lambda equals 3 space b a c t e r i a space p e r space cm cubed. This means that on average, we expect to find 3 bacteria in each cubic centimeter of water.
For a 10 space cm cubed sample, the expected number of bacteria, or the mean, would be lambda cross times 10 equals30 bacteria.

This is a Poisson distribution problem. The Poisson distribution gives the probability of a given number of events happening in a fixed interval of time or space, given a known average rate of occurrence. In this case, our "interval" is the volume of water open parentheses 10 space cm cubed close parentheses and the rate is lambda equals 3 bacteria per cm cubed.

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