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: [15.393, 241.112, -37.085, 72.85, -14.157, 77.763, -4.316, 223.701, -87.316]

54.216