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Lesson: Challenging Problem Solving - 28

Advanced Probability

[Page 28 of 37]

In any probability question, we must find the number of possible outcomes and compare them to the number of possible desired outcomes. Some probability questions, most often involving coin tosses, are special in that 1) each event has only two possible outcomes (i.e., a toss is either heads or tails), and 2) there are multiple events (i.e., there are five tosses in all).

Let's take a look at an example:

A fair coin is tossed 3 times. What is the probability that the coin landed tails up exactly twice?

For this question, our desired outcome is to toss 2 tails and 1 head. The number of possible desired outcomes is therefore the number of different ways in which we can toss two tails and one head in three tosses. We can draw these possibilities as follows:

 
T-T-H
T-H-T
H-T-T
 

Our number of possible outcomes is the number of different outcomes that could result from three tosses. Since each of the three tosses can be either heads or tails, the number of possible outcomes is equivalent to 2 2 2 = 8. To be precise, we can draw out our 8 possible outcomes:

 

H-H-H
T-T-T

H-H-T
T-H-H
H-T-H

T-T-H
H-T-T
T-H-T

 

Since we have three different outcomes of two tails, and eight different outcomes in total, the probability of tossing exactly two tails is . This probability fraction can also be obtained by calculating out the probabilities for each individual toss.

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