mean=21
SD=6
1 SD below the mean is 21-6=15
2 SD below the mean is 21-2(6)=9
The only number between them in the answer choices is 12..
thus Answer= 12
Let me know if i am wrong.
thx
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deviation
Source: Beat The GMAT — Problem Solving |
Try to practice other standard deviation problems, in general they are not very hard and always deal with the same kind of concepts.
