------ASIDE---------------------------
A little extra background on standard deviations above and below the mean

If, for example, a set has a standard deviation of 4, then:
1 standard deviation = 4
2 standard deviations = 8
3 standard deviations = 12
1.5 standard deviations = 6
0.25 standard deviations = 1
etc


So, if the mean of a set is 9, and the standard deviation is 4, then:
2 standard deviations ABOVE the mean = 17 [since 9 + 2(4) = 17]
1.5 standard deviations BELOW the mean = 3 [since 9 - 1.5(4) = 3]
3 standard deviations ABOVE the mean = 21 [since 9 + 3(4) = 21]
etc.
------ONTO THE QUESTION!!!-----------------------------------

The arithmetic mean and standard deviation of a certain normal distribution are 13.5 and 1.5, respectively. What value is exactly 2 standard deviations less than the mean?
2 standard deviations BELOW the mean = 13.5 - (2)(1.5)
= 13.5 - 3
= 10.5
= A

Cheers,
Brent
_________________

The value which is exactly two SD less than the mean is: mean-2*SD=13.5-2*1.5=10.5.

Answer: A.

Similar questions to practice:
math-questions-mean-score-88502.html
statistics-question-99221.html
70-75-80-85-90-105-105-130-130-130-the-list-consists-of-50908.html

Hope it helps.
_________________

13.5 - 2*1.5

= 10.50 , Answer must be (A)
_________________

Two standard deviations less than the mean is 13.5 - 2 x 1.5 = 13.5 - 3 =10.5.

Answer: A
_________________

Hello from the GMAT Club BumpBot!

Thanks to another GMAT Club member, I have just discovered this valuable topic, yet it had no discussion for over a year. I am now bumping it up - doing my job. I think you may find it valuable (esp those replies with Kudos).

Want to see all other topics I dig out? Follow me (click follow button on profile). You will receive a summary of all topics I bump in your profile area as well as via email.
_________________