Example:

By using a die, what is the probability of rolling a three on the first try?

Solution:

First, determine the number of possible outcomes.    In that case, there are 6 possible outcomes. From the stated problem, the roll is a success only if a 3 is rolled.  There is only 1 success outcome and 5 failures.  Thus,

Probability       = 1/(1+5)

= 1/6

In calculating probability, the probability of a series of independent events equals the product of probability of the individual events.