Hi sumit,

Mean is 21

SD is 6

Pat watched television below sd in b/w 1 and 2

SD1=21-6=15

SD2=15-6=9

Answer should be in b/w 9-15

only one option 12.....

Hope it helps....

Hey why is it :

SD1 =21-6=15
SD2 =15-6=9


It says SD 0f 6 ...so mean can lie between 21+6 and 21-7 ??

I know i am wrong , but please explain me the concept.

A population can be defined with a mean (in this case, hours of tv watched, or 21 hours) and a standard deviation (which is, how much the population can vary from the mean). In this case, the standard deviation is 6 hours. Meaning that people one standard deviation away from the mean (21 hours) either watch 21-6 hours of TV, or 15 hours, OR they can watch 21+6 hours of tv, which is 27 hours. 2 standard deviations away from the mean implies that 2 standard deviations from the population watches between 21-(6*2), or 9 hours of tv or 21+(6*2), or 33 hours of tv. Remeber this applies to the whole population. But the problem only cares about that one person, who watches between 1 and 2 standard deviations BELOW the mean. that is he watches between 21-6 hours, or 21-(6*2) hours of TV, which is between 9 and 15 hours. Hope i helped

p.s. if you ever have trouble with anything statistics related, try visualizing a bell curve, it always helps me.