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: [217.978, 70.977, 31.129, -87.408, -99.013, 249.862, -87.151, 207.527, 43.432]

60.815