to answer this question, we must first understand what it is asking.
A person's individual score on an exam is presumed to be within a distribution of scores with a mean (average) and a standard deviation (average variation from the mean of all the scores).
For a class of 30 people, the exam mean was 50, and the standard deviation was 10.
Then a score of 2 stnd dev ABOVE the mean would be: 50+2(10)=70
A score of 3 stnd dev BELOW the mean would be: 50-3(10)=20
It is not important to know the exact numbers for the mean and standard deviation for the answer, since it is asking for an explanation. The point is to find out if you understand the relationship between mean and standard deviation.