Question

Problem 5. Utility Bills The monthly utility bills in a city are normally distributed and
represented by the variable X, with a mean of $100 and a standard deviation of $12. Find the
probability that a randomly selected utility bill is
(a) less than $70,
(b) between $90 and $120,
(c) more than $140.
(2 points)
(2 points)
(2 points)
Hint: Convert the normal distribution X to Standard normal using Z formula Z =
and then look the Z-values from the table and then find the probability.
X-μ
6

Answer 1

The probabilities in the question are

• 0.0062

• 0.7499
• 0.000429

How to solve for the probabilities

a. For  x < 70

we have

z< 70 - 100/12

= z < -30/12

= -2.5

Such that p (x<70) = 0.0062

Hence the probability that is is less than $70 = 0.0062

b.  between $90 and $120,

90 - 100/12. 120 - 100/12

= -0.8333 <z< 1.67

p(90<x<120) = 0.95224 - 0.20234

= 0.7499

0.7499 is the probability of  between $90 and $120.

c. more than $140

140-100/12

= P(Z>3.3333)

= 0.000429

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