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The time between arrivals of customers at the drive-up window of a bank follows an exponential probability distribution with a mean of 10 minutes.

A. What is the probability that the arrival time between customers will be 7 minutes or less?
B. What is the probability that the arrival time between customers will be between 3 and 7 minutes?

1 Answer

6 votes

Answer:

a) 50.34% probability that the arrival time between customers will be 7 minutes or less.

b) 24.42% probability that the arrival time between customers will be between 3 and 7 minutes

Explanation:

Exponential distribution:

The exponential probability distribution, with mean m, is described by the following equation:


f(x) = \mu e^(-\mu x)

In which
\mu = (1)/(m) is the decay parameter.

The probability that x is lower or equal to a is given by:


P(X \leq x) = \int\limits^a_0 {f(x)} \, dx

Which has the following solution:


P(X \leq x) = 1 - e^(-\mu x)

The probability of finding a value higher than x is:


P(X > x) = 1 - P(X \leq x) = 1 - (1 - e^(-\mu x)) = e^(-\mu x)

Mean of 10 minutes:

This means that
m = 10, \mu = (1)/(10) = 0.1

A. What is the probability that the arrival time between customers will be 7 minutes or less?


P(X \leq x) = 1 - e^(-\mu x)


P(X \leq 7) = 1 - e^(-0.1*7) = 0.5034

50.34% probability that the arrival time between customers will be 7 minutes or less.

B. What is the probability that the arrival time between customers will be between 3 and 7 minutes?


P(3 \leq X \leq 7) = P(X \leq 7) - P(X \leq 3)


P(X \leq x) = 1 - e^(-\mu x)


P(X \leq 7) = 1 - e^(-0.1*7) = 0.5034


P(X \leq 3) = 1 - e^(-0.1*3) = 0.2592


P(3 \leq X \leq 7) = P(X \leq 7) - P(X \leq 3) = 0.5034 - 0.2592 = 0.2442

24.42% probability that the arrival time between customers will be between 3 and 7 minutes

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