:3a. Solution.  The SD of the error box is unknown, so we use
the t-distribution.  The sample average is

 	
 15+17/2 = 16 . 

The SD of the sample is 1.  The SD+ of the sample is the square
root of 2.  The SE of the sum of the measurements is estimated by

 	
 the square root of 2  x  the square root of 2  =  2. 

The SE of the average of the measurements is 2/2 = 1.  We use the
t-distribution with 2-1 = 1 degrees of freedom.  The appropriate
multiplier is 12.71, so a 95% confidence interval is given by

 	
            16 +/  12.71x1 , 

or (approximately) from 3 to 29.