That look like an obvious decreasing trend. From the first and last point we get a slope of (9-31)/(92-26) = -1/3 and a y intercept of 31 - (-1/3)26 = 39.67.
The first-serve percentage of a
tennis player in a match is normally distributed with a standard deviation of
4.3%. If a sample of 15 random matches of
the player is taken, the mean
first-serve percentage is found to be 26.4%. The margin of error of the sample
mean is 83.71%.
