Math 141-copyright Joe Kahlig, 10B
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Section 7.4: Use of Counting Techniques in Probability
Example: A type of license plate has three letters followed by two digits. If one of these plates is
select at random, find the probability that
A) The license plate has all vowels.
B) The letters are selected from the set {a, b, c, d, e}, the digit zero is not used and no letter is
repeated.
Example: A 6 sided die is rolled 5 times. What is the probability that exactly 3 of the rolls are a 4.
Example: A box contains 7 red, 6 green, and 5 black balls. What is the probability that in a sample
of 6 that
A) all the balls are the same color.
B) exactly three red and at least 2 green balls were chosen?
Math 141-copyright Joe Kahlig, 10B
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Example: A standard deck of cards is shuffled and then 5 cards are dealt.
A) What is the probability of getting at least 1 heart?
B) What is the probability of the cards being all one suit?
Example: An exam consist of 25 questions in which 12 of them must be answered. What is the
probability that a student answered exactly 2 of the first 5 questions and at least 4 of the last 5
questions?
Example: Find the probability that in a group of 7 people that all were born on a different days of
the year.
Example: Find the probability that at least two people in a group of 7 were born on the same day.
Example: Find the probability that in a group of 15 people, that at least two people were born in the
same month. Assume that months are equally likely.