Definition: In this task you will be given a list of numbers and you need to find the mean (average) of that list. The mean of a list can be found by summing every number in the list then dividing the result by the size of that list. The output should be rounded to 3 decimal places.
Input: [3.21, 2.783, 55.602, 189.097, 111.357, -44.728, 209.913, -20.324, -71.218]

48.41