S am ple Test Problems Page | 10 39. Suppose that an instructor of a math class

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S am ple Test Problems Page | 10 39. Suppose that an instructor of a math class assigns grades as follows. Students with z-scores of 1.28 or above will receive an A; z-scores between 67 and 1.28 will receive a B; z-scores between -25 and .67 will receive a C; z-scores between-84 and -.25 will receive a grade of D. Students with z-scores below-84 will receive an F What percentage of students receive a grade of A? a grade of B? a grade of C? a grade of D? a grade of F? 40. The life of a cell-phone battery is normally distributed with a mean life of 30 months and a standard deviation of 4 months. a. Identify the population parameters of interest b. What is the probability that a cell-battery will give out within the first two years? c. If 20 such batteries are taken at random, what is the probability that the average life of these 20 batteries will exceed two years? 41. The following problem refers to a sampling distribution for proportions where the rmal model with mean p and standard deviation

Explanation / Answer

39.

We will use the Z distribution table to solve the graph:

P(Z>=1.28) will get A
P(.67<Z<1.28) will get B
P(-.25<X<.67) will get C
P(-.84<Z<-.25) will get D
P(X<=-.84) will get F

Percentage of people getting A: P(X>=1.28) = 1-.8997 = .1003
Percentage of people getting B: P(.67<X<1.28) = .8997-.7486 = .1511
Percentage of people getting C: P(-.25<X<.67) = .7486-.4013 = .3473
Percentage of people getting D: P(-.84<X<-.25) = .4013-.2033 = .1980
Percentage of people getting F: P(Z<=-.84) = .2033

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