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.