20 is the mean of all the numbers in that sample of data. Lets draw a number line for the number 20.
|-----------20-----------|
Standard deviation of 3 means that all the values in that sample vary between the range (20-3) to (20+3) i.e. between 17 to 23.
|-------17----20----23-------|
Now, if standard deviation is 3 then 2.5 times the standard deviation is 7.5. So, we need to find which value lies outside of 7.5 from the mean i.e. which values lie outside of the range 20+7.5 to 20-7.5. Can be written as 12.5 to 27.5 The only one that lies outside this range from among the answer choices is 12 hence A
