Standard Deviation

between 1 & 2 std below will be 21 - 6 and 21 - 12
ie., between 15 and 9

D

This question works on normal distribution. Our mean is 21 with a 6 hour standard deviation. So we start to build a chart. 21 in the middle and then start going up by 6 and down by 6. Should look like this:

3 9 15 21 27 33 39

So, the question states that Pat watched 1 and 2 devations BELOW the mean. 15 is 1 deviation below and 9 is 2 deviations below.

The trap here is that they say 1 and two, so we can think of it as between 1 and 2, which is the middle point between 9 and 15, so 12.

I hope this helps. I was very confused when I read this too, but really when it is broken down it is easy. No math, no fomula, no problem.

John

The residents of Town X participated in a survey to determine the number of hours per week each resident spent watching television. The distribution of the results of the survey had a mean of 21 hours and a standard deviation of 6 hours. The number of hours that Pat, a resident of town X, watched television last week was between 1 and 2 standard deviations below the mean. Which of the following could be the number of hours that Pat watched television last week ?

1. 30
2. 20
3. 18
4. 12
5. 6

Mean = 21 hours
Standard Deviation = 6 hours
Pat watched television between (mean - 2*standard deviation) and (mean - 1*standard deviation)
between (21- 2*6) and (21 - 1*6)
between 9 and 15
Thus only possible time among all the options is 12
IMO D