MATH 160 SECTION 2
Instructor: Selin Kalaycioglu PRACTICE TEST # 3
1) In a large Metropolitan area, 20% of the families have an ad-
justed gross income of $80,000 or more reported on their local income
tax return. A random audit chooses 100 of these returns for careful
study. Let X be the number of local income tax returns audited that
show an adjusted gross income of under $80,000.
a) Find the mean of X. (Answer 80)
b) What is the probability that at least 30 of the returns au-
dited show an adjusted gross income of more than $80,000? (Answer
0.0062)
2) People with type O-negative blood are universal donors whose
blood can safely be given to anyone. Only 7.2% of the population has
O-negative blood. A mobile blood center is visited by 20 donors in
the afternoon. Let X denote the number of universal donors among
them.
a) Find the mean of X. (Answer 1.44)
b) Find the standard deviation of X. (Answer 1.15)
c) Find the probability that Xis at least 2. (Answer 0.427)
d) Now you do a larger study with 1000 donors. What is the
probability that Xis at least 200.
3) Scores on a University exam are normally distributed with a
mean of 68 and a standard deviation of 9. Using the 68-95-99.7 rule,
what percentage of students score above 77? (Answer 16%)
4) The time to complete a standardized exam is approximately
normal with a mean of 70 minutes and a standard deviation of 10
minutes. Using the 68-95-99.7 rule, if students are given 90 minutes
to complete the exam, what percentage of students will not finish?
(Answer 2.5%)
5) Birthweights at a local hospital have a normal distribution with
a mean of 110 oz. and a standard deviation of 15 oz.
