The mean is the sum of all values divided by the number of values:

\(\displaystyle\overline{{x}}={\frac{{{11}+{37}+{54}+{70}+{93}}}{{{5}}}}={\frac{{{265}}}{{{5}}}}={53}\)

Predictions for z-values closest to the sample mean & are the most reliable. The more reliable the prediction, the narrower the confidence interval will be.

The widest confidence interval is then for the c-value that is furthest from the sample mean © = 53, which is the value 72 MPa.

The narrowest confidence interval is then for the x-value that is closest to the sample mean \(\displaystyle\overline{{x}}={53}\), which is the sample mean of 53 MPa itself