The instructor wants to give an A to the students whose scores were in the top of the class. What is the minimum score needed to get an A

The minimum svore required to get an A is 85.3.

Step-by-step explanation:

Complete Question

Exam grades: Scores on a statistics final in a large class were normally distributed with a mean of 75 and a standard deviation of 8.

The instructor wants to give an A to the students whose scores were in the top 10% of the class. What is the minimum score needed to get an A?

Solution

Scores in the top 10% of the class will have a minimum greater than the remaining bottom 90% of the class.

If the minimum score for the top 10% of the class is x'

P(X ≤ x') = 90% = 0.90

If the z-score of this minimum score of the top 10%, x', is z'.

P(X ≤ x') = P(z ≤ z') = 0.90

using the z-distribution tables

z' = 1.282

But the z-score of any value is given as the value minus the mean divided by the standard deviation.

z = (x - μ)/σ

So,

z' = (x' - μ)/σ

Mean = 75

Standard deviation = 8

z' = 1.282

1.282 = (x' - 75)/8

x' = (1.282 × 8) + 75 = 85.256

= 85.3 to 3 s.f.

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