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: [239.95, 218.144, 144.395, 209.674, 171.51, 99.035, 225.941, 32.722, -21.807]

146.618