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Example: New Bank Accounts
Suppose that on the average two new accounts per day are opened at an Imperial Savings Branch
bank. Let’s find the probability that on a given day at least 7 new accounts are opened. The
complement of opening at least 7 new accounts is opening at most 6 new accounts, which has
probability poissoncdf(2, 6) = 0.9955. The probability of opening at least 7 new accounts is
1−0.9955 = 0.0045.
Geometric Probabilities
Your calculator can compute probabilities for a geometric random variable with probability of
success p using the geometpdf( command, located on the DISTR page. To find the probability of
the random variable taking the value x, type geometpdf(p, x).
Example: Car Ignition
Suppose that a car with a bad starter can be started 90% of the time by turning on the ignition.
What is the probability that it will take three tries to get the car started? Type geometpdf(0.9, 3);
the answer is 0.9%.
Cumulative Geometric Probabilities
As with the binomial and cumulative probability functions, there is a cumulative version
geometcdf( . It can be used to find the probability that a geometric random variable will take a
value of at most x by typing geometcdf(p, x).
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