input: Write a function to calculate the average of a list of numbers.
  assert calc_avg([5, 10, 15, 20, 25]) == 15
  assert calc_avg([1, 2, 3, 4, 5]) == 3
  assert calc_avg([10, 20, 30, 40, 50]) == 30
output: def calc_avg(numbers):
      return sum(numbers) / len(numbers) if numbers else 0