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Exercises 1

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Faculty of Basic Sciences Probability and Statistics exercises

Department of Mathematics

1. In a school, 48% of the students take a foreign language class and 19% of
students take both foreign language and technology. What is the probability that
a student takes technology given that the students takes foreign language?
2. The test contains 10 questions, each one with available four different answers,
among which just one is correct. To pass the test at least 5 questions must be
answered correctly. What is the probability that completely unprepared student
will pass the test ?
3. In the class of 30 students, seven of them don't have done the homework. The
teacher choosed randomly 6 students. What is the chance that at least four of
them have done their homework ?
4. Three shooters shoot at the same target, each of them shoots just once. The first
one hits the target with a probability of 70%, the second one with a probability
of 80% and the third one with a probability of 90%. What is the probability that
the shooters will hit the target
a) at least once
b) at least twice ?
5. Based on incidence rate, the following table presents the corresponding numbers
per 100,000 people.

Sympto Cancer
Total
m No Yes

No 99989 0 99989

Yes 10 1 11

Total 99999 1 100000


Which can then be used to calculate the probability of having cancer when you
have the symptoms:
6. A factory produces an item using three machines—A, B, and C—which account
for 20%, 30%, and 50% of its output, respectively. Of the items produced by
machine A, 5% are defective; similarly, 3% of machine B's items and 1% of
machine C's are defective. If a randomly selected item is defective, what is the
probability it was produced by machine C?
7. X is a discrete random variable. The table below defines a probability
distribution for X

X 0 1 2 3
P 0.17 0.14 0.36 0.33

What is the expected value of X? What is the variance value of X?


8. The random variable X is given by the following PDF. Check that this is a valid
PDF and calculate the expected, the variance, the standard deviation values of X.

9. Let X be a continuous random variable with the following


−x
f ( x )= c e if x ≥ 0
{ 0 otherwise

where c is a positive constant.


a. Find c.
b. Find the cumulative distribution function of X.
c. Find P(1<X<3).

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