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oee [108]
3 years ago
6

f) The life of a power transmission tower is exponentially distributed, with mean life 25 years. If three towers, operated indep

endently, are being erected at the same time, what is the probability that at least 2 will still stand after 35 years?
Mathematics
1 answer:
Step2247 [10]3 years ago
4 0

Answer:

15.24% probability that at least 2 will still stand after 35 years

Step-by-step explanation:

To solve this question, we need to understand the binomial distribution and the exponential distribution.

Binomial probability distribution

The binomial probability is the probability of exactly x successes on n repeated trials, and X can only have two outcomes.

P(X = x) = C_{n,x}.p^{x}.(1-p)^{n-x}

In which C_{n,x} is the number of different combinations of x objects from a set of n elements, given by the following formula.

C_{n,x} = \frac{n!}{x!(n-x)!}

And p is the probability of X happening.

Exponential distribution:

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

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

In which \mu = \frac{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}

Probability of a single tower being standing after 35 years:

Single tower, so exponential.

Mean of 25 years, so m = 25, \mu = \frac{1}{25} = 0.04

We have to find P(X > 35)

P(X > 35) = 1 - P(X \leq x) = 1 - (1 - e^{-\mu x}) = e^{-0.04*35} = 0.2466

What is the probability that at least 2 will still stand after 35 years?

Now binomial.

Each tower has a 0.2466 probability of being standing after 35 years, so p = 0.2466

3 towers, so n = 3

We have to find:

P(X \geq 2) = P(X = 2) + P(X = 3)

In which

P(X = x) = C_{n,x}.p^{x}.(1-p)^{n-x}

P(X = 2) = C_{3,2}.(0.2466)^{2}.(0.7534)^{1} = 0.1374

P(X = 3) = C_{3,3}.(0.2466)^{3}.(0.7534)^{0} = 0.0150

P(X \geq 2) = P(X = 2) + P(X = 3) = 0.1374 + 0.0150 = 0.1524

15.24% probability that at least 2 will still stand after 35 years

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