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: [139.238, 225.164, -21.563, -22.339, -77.136, 96.772, -40.208, 121.974, 228.852]

72.306