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.
[-51.986, -35.463, 117.863, 85.376, 134.932, 22.087, -21.652, 229.811, -55.739, 215.347]