Hey there,

This is very easy one. All you have to do is just ad/subtract SD to/from the mean. Before you have to multiply by the given number, which is 2.5 SD. Thus,
20 + (2.5 * 3) = 20 + 7.5 = 27.5
20 - (2.5 * 3) = 20 - 7.5 = 12.5.
Your data should be outside the range of 12.5 ~ 27.5. All the numbers lie in the range except 12, the answer must be [A].

rainbownlife wrote:please explain. Waht are all the SD formule one needs to remmeber.

thanx in advance !

If a sample data has mean = 20 and std deviation=3.0 , which of the following is more than 2.5 std deviation from mean.

12,13.5,17,23.5,26.5

Smushka's explanation is 100% correct, I just want to add a strategic guessing tip for this question.

Since the question is asking us for the number outside of 2.5 SD, the only possible choices are the two extremes, (a) and (e) - so, even if you're stuck, you have a 50/50 shot at the question.

great!
I just reviewed SD basics...and now this question makes sense
thanks for your replies !