Page 75 - Probability Demystified

64 CHAPTER 4 The Multiplication Rules

PRACTICE

1. In a study, there are 8 guinea pigs; 5 are black and 3 are white. If
2 pigs are selected without replacement, find the probability that both
are white.
2. In a classroom there are 8 freshmen and 6 sophomores. If three
students are selected at random for a class project, find the probabil-
ity that all 3 are freshmen.
3. Three cards are drawn from a deck of 52 cards without replacement.
Find the probability of getting 3 diamonds.
4. A box contains 12 calculators of which 5 are defective. If two calcu-
lators are selected without replacement, find the probability that both
are good.
5. A large flashlight has 6 batteries. Three are dead. If two batteries are
selected at random and tested, find the probability that both are dead.

ANSWERS
3 2 1 3
1. P(white and white) ¼ ¼
8 4 7 28
8 7 1 6 1 2
2. P(3 freshmen) ¼ 2 2 ¼
14 13 12 13
1
13 12 11 11
3. P(3 diamonds) ¼ 4 ¼
52 51 50 850
7 6 1 7
4. P(2 good) ¼ 2 ¼
12 11 22
3 2 1 3 1
5. P(2 batteries dead) ¼ ¼ ¼
6 3 5 15 5

Conditional Probability

Previously, conditional probability was used to find the probability of
sequential events occurring when they were dependent. Recall that P(B|A)
means the probability of event B occurring given that event A has already
occurred. Another situation where conditional probability can be used is
when additional information about an event is known. Sometimes it might be