Posted by Rhea on .
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,
14. Variance = SD^2
Z = (scoremean)/SD = (score62)/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.