Answer:
a) 0.1496 = 14.96% probability that the demand will exceed 190 cfs during the early afternoon on a randomly selected day.
b) Capacity of 252.6 cubic feet per second
Step-by-step explanation:
Exponential distribution:
The exponential probability distribution, with mean m, is described by the following equation:
![f(x) = \mu e^{-\mu x}](https://tex.z-dn.net/?f=f%28x%29%20%3D%20%5Cmu%20e%5E%7B-%5Cmu%20x%7D)
In which
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](https://tex.z-dn.net/?f=P%28X%20%5Cleq%20x%29%20%3D%20%5Cint%5Climits%5Ea_0%20%7Bf%28x%29%7D%20%5C%2C%20dx)
Which has the following solution:
![P(X \leq x) = 1 - e^{-\mu x}](https://tex.z-dn.net/?f=P%28X%20%5Cleq%20x%29%20%3D%201%20-%20e%5E%7B-%5Cmu%20x%7D)
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}](https://tex.z-dn.net/?f=P%28X%20%3E%20x%29%20%3D%201%20-%20P%28X%20%5Cleq%20x%29%20%3D%201%20-%20%281%20-%20e%5E%7B-%5Cmu%20x%7D%29%20%3D%20e%5E%7B-%5Cmu%20x%7D)
The operator of a pumping station has observed that demand for water during early afternoon hours has an approximately exponential distribution with mean 100 cfs (cubic feet per second).
This means that ![m = 100, \mu = \frac{1}{100} = 0.01](https://tex.z-dn.net/?f=m%20%3D%20100%2C%20%5Cmu%20%3D%20%5Cfrac%7B1%7D%7B100%7D%20%3D%200.01)
(a) Find the probability that the demand will exceed 190 cfs during the early afternoon on a randomly selected day. (Round your answer to four decimal places.)
We have that:
![P(X > x) = e^{-\mu x}](https://tex.z-dn.net/?f=P%28X%20%3E%20x%29%20%3D%20e%5E%7B-%5Cmu%20x%7D)
This is P(X > 190). So
![P(X > 190) = e^{-0.01*190} = 0.1496](https://tex.z-dn.net/?f=P%28X%20%3E%20190%29%20%3D%20e%5E%7B-0.01%2A190%7D%20%3D%200.1496)
0.1496 = 14.96% probability that the demand will exceed 190 cfs during the early afternoon on a randomly selected day.
(b) What water-pumping capacity, in cubic feet per second, should the station maintain during early afternoons so that the probability that demand will exceed capacity on a randomly selected day is only 0.08?
This is x for which:
![P(X > x) = 0.08](https://tex.z-dn.net/?f=P%28X%20%3E%20x%29%20%3D%200.08)
So
![e^{-0.01x} = 0.08](https://tex.z-dn.net/?f=e%5E%7B-0.01x%7D%20%3D%200.08)
![\ln{e^{-0.01x}} = \ln{0.08}](https://tex.z-dn.net/?f=%5Cln%7Be%5E%7B-0.01x%7D%7D%20%3D%20%5Cln%7B0.08%7D)
![-0.01x = \ln{0.08}](https://tex.z-dn.net/?f=-0.01x%20%3D%20%5Cln%7B0.08%7D)
![x = -\frac{\ln{0.08}}{0.01}](https://tex.z-dn.net/?f=x%20%3D%20-%5Cfrac%7B%5Cln%7B0.08%7D%7D%7B0.01%7D)
![x = 252.6](https://tex.z-dn.net/?f=x%20%3D%20252.6)
Capacity of 252.6 cubic feet per second