Question

in a study reported in the Flurry Blog on Oct. 29, 2012, the mean age of tablet users was 34 yrs, with a standard deviation of 15 years. Assuming a normal distribution, what is the approximate probability of picking a random sample of 40 tablet users with a mean age between 31 and 35 yrs?

		.56, between z = +1.27 and -0.42
		.56, between z = 31 and 35
		.56, between z = -1.27 and +0.42
		.56 between z = -2.27 and +0.42
		none of the above

Answer 1

Given that the mean age of tablet users was = 34 yrs, with a standard deviation of = 15 years. Assuming a normal distribution the probability of mean age between = 31 and = 35 years is calculated by finding the Z score at 31 and 35 as:

and at Mean = 35

Thus the probability P(31<<35) = P(-1.27 < Z < 0.42) the probability of a normal distribution can be calculated using the excel formula for normal distribution which is =NORM.S.DIST(0.42,TRUE)-NORM.S.DIST(-1.27,TRUE), thus the probability is computed as 0.56.

So, the correct answer would be:

0.56, between z = -1.27 and +0.42