PROBABILITY OF SUCCESS
If a trial must result in any of n equally likely ways, and if s is the number of successful ways and f is the number of failing ways, the probability of success is
EXAMPLE: What is the probability that a coin will land heads up?
SOLUTION: There is only one way the coin can land heads up; therefore, s equals 1. There is also only one way the coin can land other than heads up; therefore, f equals 1. Since
then the probability of success is
This, then, is the ratio of successful ways in which the trial can succeed to the total number of ways the trial can result.
EXAMPLE: What is the probability that a die (singular of dice) will land with a 3 showing on the upper face?
SOLUTION: A die has a total of 6 sides. Therefore the die can land with a 3 face up 1 favorable way and 5 unfavorable ways.
EXAMPLE: What is the probability of drawing a black marble from a box of marbles if all 6 of the marbles in the box are white?
SOLUTION: There are no favorable ways of success and there are 6 total ways. Therefore,
EXAMPLE: What is the probability of drawing a black marble from a box of 6 black marbles?
SOLUTION: There are 6 successful ways and no unsuccessful ways of drawing the marble. Therefore,
The previous two examples are the extremes of probabilities and intuitively demonstrate that the probability of an event ranges from 0 to 1 inclusively.
EXAMPLE: A box contains 6 hard lead pencils and 12 soft lead pencils. What is the probability of drawing a soft lead pencil from the box?
SOLUTION: We are given
1. What is the probability of drawing an ace from a standard deck of 52 playing cards?
2. What is the probability of drawing a black ace from a standard deck of playing cards?
3. If a die is rolled, what is the probability of an odd number showing on the upper face?
a. he will draw a nickel?
b. he will draw a half-dollar? c. he will draw a quarter?