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: [-13.371, 112.6, 240.359, 165.509, -33.503, 118.087, 172.774, -1.423, 20.807]

86.871