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Mathematics and Statistics
MAST20004
Probability
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Cover image: Jakob Bernoulli (1654 - 1705, Basel)
Jakob Bernoulli was a Swiss mathematician who made significant contributions to algebra,
calculus, mechanics, calculus of variations, geometry, infinite series, and probability. His
most renowned work was the derivation of the law of large numbers, which put simply, states
that the average proportion of times an event occurs approaches the theoretical probability
of the event, as the number of trials increases.
This compilation has been made in accordance with the provisions of Part VB of the copyright
act for the teaching purposes of the University.
For use of students of the University of Melbourne enrolled in the second year subject
MAST20004 Probability.
MAST20004 Probability
Problem Sheet 1
Axioms of Probability
1. Sample Space
Define a sample space for the experiment of putting three different books on a shelf in random
order. If two of these three books are a two-volume dictionary, describe the event that these
volumes stand in increasing order side-by-side (i.e., volume I precedes volume II and next to
each other).
2. Events
Let E, F , and G be three events ; explain the meaning of the two relations E ∪F ∪G = G and
E ∩ F ∩G = G.
3. More Events
Prove that the event B is impossible if and only if for every event A,
A = (B ∩ Ac) ∪ (Bc ∩ A).
4. Cards
In an experiment, cards are drawn, one by one, at random and successively from an ordinary
deck of 52 cards. Let An be the event that no face card or ace appears on the first n− 1 draws,
and the nth draw is an ace. In terms of Ans, find an expression for the event that an ace appears
before a face card, if
(a) the cards are drawn with replacement ;
(b) they are drawn without replacement.
5. Event Identities
Let A and B be two events. Prove the following relations by the elementwise method.
(a) (A\(A ∩B)) ∪B = A ∪B ;
(b) (A ∪B)\(A ∩B) = (A ∩Bc) ∪ (Ac ∩B).
6. Infinite Sequence of Sets
Let {A1, A2, A3, . . .} be a sequence of events of a sample space S. Find a sequence {B1, B2, B3, . . .}
of mutually exclusive events such that for all n ≥ 1, ∪ni=1Ai = ∪ni=1Bi.
1
7. Hiring
A company has only one position with three highly qualified applicants : John, Barbara, and
Marty. However, because the company has only a few women employees, Barbara’s chance to
be hired is 20% higher than John’s and 20% higher than Marty’s. Find the probability that
Barbara will be hired.
8. Probability Statements
Which of the following statements is true ? If a statement is true, prove it. If it is false, give a
counterexample.
(a) If P(A) + P(B) + P(C) = 1, then A, B, and C are mutually exclusive events ;
(b) If P(A ∪B ∪ C) = 1, then A, B, and C are mutually exclusive events.