Statistics
posted by robyn .
On a chemistry exam, Student A scored a 81 and Student B scored a 70. The mean for the class was 75 and the standard deviation was 3.8.
What percentage of students scored in somewhere in between Student A and Student B?

about 15 percent if you believe it was a normal distribution. Seldom it is.
http://davidmlane.com/hyperstat/z_table.html 
Here is another method.
If it approximates a normal distribution, you can use
Z = (scoremean)/SD
Find table in the back of your statistics text labeled something like "areas under normal distribution" to find the proportions related to the Z scores.
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