1. The problem asks for the population mean height of 9 grandchildren, given their heights in inches: 66, 76, 79, 73, 66, 69, 76, 70, 75. 2. The population mean ($\mu$) is calculated by summing all data points and dividing by the number of data points. 3. First, sum the heights: $$ 66 + 76 + 79 + 73 + 66 + 69 + 76 + 70 + 75 = 650 $$ 4. There are 9 grandchildren, so the mean is: $$ \mu = \frac{650}{9} \approx 72.2 $$ 5. Rounded to the nearest tenth, the population mean height is 72.2 inches. Final answer: 72.2