1. The independent and normally distributed random variables
X and
Y have known standard deviations of

and

, respectively. A sample of 36 observations from the
X population yielded

, and a sample of 49 observations from the
Y population resulted in

. Using
Equation (11.4), find a 99% confidence interval for the mean difference

.
2. The American Automobile Assn. (AAA) compared the mean prices per gallon of gasoline in two different metropolitan areas. The mean price per gallon in area
X was

(
nX = 40) and the mean price per gallon in area
Y was

(
nY = 35). If prior studies reveal that

and , find a 95% confidence interval for the mean difference . What assumptions are being made?