input: Write a python function to find the average of all numbers in an array.
  assert average([1, 2, 3, 4, 5]) == 3.0
  assert average([10, 20, 30, 40, 50]) == 30.0
  assert average([7, 14, 21, 28, 35]) == 21.0
output: def average(arr):
      return sum(arr) / len(arr)