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

15+17/2 = 16 .  <p>

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 <p>

the square root of 2  x  the square root of 2  =  2.  <p>

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 <p>

16 +/  12.71x1 ,  <p>

or (approximately) from 3 to 29. <p>