Homework Help: Statistics - M/C

Posted by Rhea on Saturday, October 23, 2010 at 7:37pm.

14.
The marks on a statistics test are normally distributed with a mean of 62 and a variance of 225. If the instructor wishes to assign B’s or higher to the top 30% of the students in the class, what mark is required to get a B or higher?

a. 68.7 b. 71.5 c. 73.2 d. 74.6 e.69.9

The answer is E, but don't know why.

2. Suppose the test scores of 600 students are normally distributed with a mean of 76 and variance
of 64. The number of students scoring between 70 and 82 is:

a. 272 b. 164 c. 260 d. 136 e. 328

The answer is E but don't know why.

Statistics - M/C - PsyDAG, Saturday, October 23, 2010 at 8:38pm
14. Variance = SD^2

Z = (score-mean)/SD = (score-62)/15

Find table in the back of your statistics text labeled something like "areas under normal distribution" to find the proportion (.30) to give you the Z score. Put that value in the above equation to find the score.

Z = ([70 or 82]-76)/8

Use the Z scores in the same table to find the proportions between them and multiply by 600.

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