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.
[13.828, 155.859, 207.974, -44.32, 120.722, 115.533, -23.042, 41.755, 76.642]