the average speed in mph of 20 randomly selected qualifiers for a race was 215.167 mph with a standard deviation of 3.12 mph. estimate the average qualifying speed with a 95% confidence interval.

Answer 1

You need 4 pieces of info to calculate this, lucky they are all provided.

1. Number of observations: n = 20
2. Mean: X = 215.167
3. Standard Deviation: s = 3.12

The 95% CI translates into a Z value of 1.960 (from table) you can also solve for it, but it is not needed for this problem.

now use the following:

X +/- Z (s / sqrt n)

this will give you the answer.