1. **State the problem:** We are given a list of numbers and want to find the mean (average) of these numbers. 2. **Formula:** The mean of a set of numbers $x_1, x_2, \ldots, x_n$ is given by $$\text{Mean} = \frac{\sum_{i=1}^n x_i}{n}$$ where $n$ is the total number of values. 3. **Count the numbers:** There are 25 numbers in the list. 4. **Sum the numbers:** $$11.5 + 10.8 + 12.2 + 9.5 + 23.6 + 27.0 + 21.6 + 5.4 + 14.9 + 10.8 + 17.2 + 11.5 + 20.3 + 13.5 + 17.6 + 16.2 + 18.9 + 10.8 + 32.4 + 15.5 + 5.0 + 9.0 + 9.5 + 7.0 + 9.0 = 388.8$$ 5. **Calculate the mean:** $$\text{Mean} = \frac{388.8}{25}$$ 6. **Simplify the fraction:** $$\text{Mean} = \cancel{\frac{388.8}{25}} = 15.552$$ 7. **Final answer:** The mean of the given numbers is **15.552**.