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Placement Preparation
**Specialization:
**Quantitative Aptitude**Offline Downloads:
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Permutations &
Combinations
Extension 1 Mathematics
HSC Revision

Multiplication Rule
If one event can occur in m ways, a second
event in n ways and a third event in r, then
the three events can occur in m × n × r ways.
Example Erin has 5 tops, 6 skirts and 4 caps
from which to choose an outfit.
In how many ways can she select one top, one
skirt and one cap?
Solution: Ways = 5 × 6 ×
4

Repetition of an Event
If one event with n outcomes occurs r times with
repetition allowed, then the number of ordered
arrangements is nr
Example 1 What is the number of arrangements if a die is
rolled
(a) 2 times ?
6×6
= 62
(b) 3 times ?
6×6×6
= 63
(b) r times ? 6 × 6 × 6 × ……. = 6r

Repetition of an Event
Example 2
(a) How many different car number plates are
possible
with 3 letters followed by 3 digits?
Solution: 26 × 26 × 26 × 10 × 10 × 10 = 263 × 103
(b) How many of these number plates begin with ABC
?
Solution: 1 × 1 × 1 × 10 × 10 × 10 = 103
(c) If a plate is chosen at random, what is the
probability that it begins with ABC?
Solution:
103
263 × 103
=
1 .
263

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