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21 April, 23:22

There are 46 students in an elementary statistics class. On the basis of years of experience, the instructor knows that the time needed to grade a randomly chosen first examination paper is a random variable with an expected value of 5 min and a standard deviation of 4 min. (Round your answers to four decimal places.) (a) If grading times are independent and the instructor begins grading at 6:50 P. M. and grades continuously, what is the (approximate) probability that he is through grading before the 11:00 P. M. TV news begins? (b) If the sports report begins at 11:10, what is the probability that he misses part of the report if he waits until grading is done before turning on the TV?

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  1. 22 April, 00:32
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    a) P (T < 250 mins) = 0.7695

    b) P (T > 260 mins) = 0.1344

    Step-by-step explanation:

    - The RV from a sample has the following parameters that are mean = 5 mins, and standard deviation s = 4 mins.

    - The entire population has n = 46 students.

    - We will first compute the population mean u and population standard deviation σ as follows:

    u = n*mean

    u = 46*5 = 230 mins

    σ = sqt (n) * s

    σ = sqt (46) * 4

    σ = 27.129 mins

    - Approximating that the time taken T to grade the population of entire class follows a normal distribution with parameters u and σ as follows:

    T~ N (230, 27.129)

    Find:

    - If grading times are independent and the instructor begins grading at 6:50 P. M. and grades continuously, what is the (approximate) probability that he is through grading before the 11:00 P. M. TV news begins?

    - The total time till 6:50 PM to 11:00 PM is (4 hr and 10 mins) = 250 mins.

    - We will compute the Z-value as follows:

    Z = (250 - 230) / 27.129

    Z = 0.7372

    - Then use the Z-Tables and determine the probability:

    P (T < 250 mins) = P (Z < 0.7372)

    P (T < 250 mins) = 0.7695

    Find:

    - If the sports report begins at 11:10, what is the probability that he misses part of the report if he waits until grading is done before turning on the TV?

    - For the teacher to miss the sports report he must take more time than 6:50 PM to 11:10 P. M.

    - The total time till 6:50 PM to 11:10 PM is (4 hr and 20 mins) = 260 mins.

    - We will compute the Z-value as follows:

    Z = (260 - 230) / 27.129

    Z = 1.10582

    - Then use the Z-Tables and determine the probability:

    P (T > 260 mins) = P (Z > 1.10582)

    P (T > 260 mins) = 0.1344
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