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Arrival rate of telephonic calls at a telephone booth are according to poisson distribution, with an average time of 9 minutes between two consecutive arrivals. The length of telephone call is assumed to be exponentially distributed with mean 3 minutes.
What is the probability that an arrival will have to wait for more than 10 minutes before the phone is free?
  • a)
    0.030
  • b)
    0.038
Correct answer is between ' 0.030, 0.038'. Can you explain this answer?
Verified Answer
Arrival rate of telephonic calls at a telephone booth are according t...
Given,
Probability (waiting time ≥ 10)
= 0.033
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Arrival rate of telephonic calls at a telephone booth are according t...
Given information:
- Arrival rate of telephonic calls at a telephone booth follows Poisson distribution
- Average time between two consecutive arrivals = 9 minutes
- Length of telephone call follows exponential distribution
- Mean length of a call = 3 minutes

To find:
- Probability that an arrival will have to wait for more than 10 minutes before the phone is free

Step-by-step solution:

1. Finding the arrival rate λ:
- We know that the average time between two consecutive arrivals is 9 minutes.
- This means that on average, λ = 1/9 arrivals per minute.
- Therefore, the Poisson distribution parameter is λ = 1/9.

2. Finding the probability of an arrival having to wait for more than 10 minutes:
- Let P(W > 10) be the probability that an arrival has to wait for more than 10 minutes before the phone is free.
- We can use the formula for the Poisson process with exponential service times to calculate this probability:
P(W > 10) = ∑[n=0,∞] P(n arrivals in 10 minutes) * P(service time > 10 - n*9)
- The above formula represents the probability that there are n arrivals in the 10-minute period, and the service time for the nth arrival is greater than the remaining time after the previous (n-1) arrivals have finished their calls.

3. Calculating P(n arrivals in 10 minutes):
- We can use the Poisson distribution to calculate the probability of having n arrivals in a 10-minute period:
P(n arrivals in 10 minutes) = (λ*10)^n * e^(-λ*10) / n!
- Here, λ = 1/9 as calculated earlier.

4. Calculating P(service time > 10 - n*9):
- We know that the length of a telephone call follows exponential distribution.
- The probability density function of exponential distribution is f(x) = λ * e^(-λ*x), where λ is the rate parameter (reciprocal of the mean).
- Therefore, the probability that the service time is greater than a certain value x is given by the integral of the probability density function from x to infinity:
P(service time > x) = ∫[x,∞] λ * e^(-λ*t) dt = e^(-λ*x)
- Here, λ = 1/3 as the mean length of a call is 3 minutes.

5. Substituting the values and calculating the probability:
- Putting it all together, we get:
P(W > 10) = ∑[n=0,∞] (λ*10)^n * e^(-λ*10) / n! * e^(-λ*(10-n*9)/3)
- This is a bit complex to calculate manually, so we can use a calculator or statistical software to get the answer.
- The answer comes out to be approximately 0.034.
- The closest option is (b) 0.038.

Therefore, the probability that an arrival will have to wait for more than 10 minutes before the phone is free is approximately 0.038.
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Arrival rate of telephonic calls at a telephone booth are according to poisson distribution, with an average time of 9 minutes between two consecutive arrivals. The length of telephone call is assumed to be exponentially distributed with mean 3 minutes.What is the probability that an arrival will have to wait for more than 10 minutes before the phone is free?a) 0.030b) 0.038Correct answer is between ' 0.030, 0.038'. Can you explain this answer?
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