Math 425 – Section 8
Homework 6 – Chapter 4, problems 53, 55, 57, 59, 60, 74(b), 75, 79
Problem 55.
A certain typing agency employs 2 typists. The average number
of errors per article is 3 when typed by the ﬁrst typist and 4.2 when typed by the
second. If your article is equally likely to be typed by either typist, approximate
the probability that it will have no errors.
Solution.
First, we condition on the typist:
P
{
no errors
}
=
P
{
no errors

1st typist
}
P
{
1st typist
}
+
P
{
no errors

2nd typist
}
P
{
2nd typist
}
We model the number of errors of the ﬁrst, resp. second, typist by a Poisson
random variable with parameter
λ
1
= 3, resp.
λ
2
= 4
.
2. So,
P
{
no errors
}
=
e

3
×
1
2
+
e

4
.
2
×
1
2
Problem 57.
Suppose that the number of accidents occurring on a highway
each day is a Poisson random variable with parameter
λ
= 3.
(a) Find the probability that 3 or more accident occur today.
(b) Repeat part (a) under the assumption that at least 1 accident occurs today.
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 Winter '08
 Buckingham
 Math, Probability, Probability theory, Poisson random variable

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