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Question: Average score of the students in a statistics contest is 65, and the standard deviation is 15. If a random sample of 16 students who participated in the contest are selected, then find the probability that the sample mean of the scores lies between 61 and 67.


A ) 15.52%
B ) 20.25%
C ) 56.02%
D ) 35.77%

Steps to derive

1 Since the variable is approximately normally distributed, the distribution of sample means will be approximately normal with a mean of 65.

2 Let X be the sample mean of the scores in statistics contest.

3 The standard deviation of the sample mean is σX = σn = 1516 = 3.75

4 The problem is to find P(61 < X < 67)

5 = P(61 - 6515/16 < X-μσ/n < 67 - 6565/16) = P(- 1.07 < z < 0.533)
 [Convert X to standard normal units.]

6 = P(- 1.07 < z < 0) + P(0 < z < 0.533) = 0.2025 + 0.3577 = 0.5602 = 56.02%.
 [Use the standard normal distribution table.]


7 The probability that the sample mean of the scores in statistics contest lies between 61 and 67 is 56.02%.



Hence the right answer is Option C

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