input: Write a function to compute the mean of a list of numbers.
  assert compute_mean([1, 2, 3, 4, 5]) == 3.0
  assert compute_mean([10, 20, 30, 40, 50]) == 30.0
  assert compute_mean([7, 8, 9, 10, 11]) == 9.0
output: def compute_mean(numbers):
      return sum(numbers) / len(numbers)