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.
[86.206, 195.457, 240.739, 94.806, -13.486, 78.413, -10.174]