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.
[249.053, 95.147, 78.214, 98.619, 235.658, -87.649, 28.294]