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Probability
Probability is another statistics-related topic you might
see on the Math IC. You should be familiar with the probability
formula and with applying the probability formula to calculate the
likely outcome of independent events.
The Probability Formula
The probability of an event is a number between
0 and 1 that represents the likelihood of that event occurring.
You can calculate the probability of an event by dividing the number of
desired outcomes by the total number of possible outcomes.
For example, in a deck of 52 cards, the probability of
pulling one of the 13 hearts from the deck is much higher than the
likelihood of pulling out the ace of spades. To calculate an exact
value for the probability of drawing a heart from the deck, divide
the number of hearts you could possibly draw by the total number
of cards in the deck.
In contrast, the possibility of drawing the single ace
of spades from the deck is:
After looking at these examples, you should be able to
understand the general formula for calculating probability. Let’s
look at a more complicated example:
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There are 3 ways for Joe to pick a green marble (since
there are 3 different green marbles), but there are 10 total possible
outcomes (one for each marble in the bag). Therefore, you can simply
calculate the probability of picking a green marble:
When calculating probabilities, always be careful to count
all of the possible favorable outcomes among the total possible
outcomes.
The Range of Probability
The probability, P, of any event occurring
will always be 0 ≤ P ≤ 1. A probability of 0 for
an event means that the event will never happen.
A probability of 1 means the event will always occur. For example,
drawing a green card from a standard deck of cards has a probability
of 0; getting a number less than seven on a single roll of one die
has a probability of 1.
If you are ever asked to calculate probability on the
Math IC, you can automatically eliminate any answer choices that
are less than 0 or greater than 1.
The Probability That an Event Will Not Occur
Some Math IC questions ask you to determine the probability
that an event will not occur. In that case, just figure out the
probability of the event occurring, and subtract that number from
1.
Probability and Multiple Events
The most difficult Math IC probability questions deal
with the probability of multiple events occurring. Such questions
will always deal with independent events—events whose probability
is not dependent on the outcome of any other event. For these questions,
the probability of both events occurring is the product of the outcomes
of each event: P(A) 
P(B),
where P(A) is the probability
of the first event and P(B) is
the probability of the second event.
P(B),
where P(A) is the probability
of the first event and P(B) is
the probability of the second event.For example, the probability of drawing a spade from a
full deck of cards and rolling a one with a six-sided
die is the product of the probability of each event.
The same principle can be applied to finding the probability
of a series of events. Take a look at the following problem:
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In order to find the probability of
three consecutive events, you should first find the probability
of each event separately. The first jellybean has a 15⁄50 chance
of being licorice-flavored. The second jellybean, however, is a
different story. There are now only 49 jelly beans left in the jar, so
the probability of getting another licorice-flavored one is 14⁄49. The
probability of getting a third licorice-flavored jellybean is 13⁄48. The
odds of getting three licorice jelly beans in a row is:
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