input: Write a function to calculate the average of a list of scores.
  assert calculate_average([90, 80, 70, 60]) == 75.0
  assert calculate_average([100, 95, 85, 75, 65]) == 84.0
  assert calculate_average([50, 50, 50, 50]) == 50.0
output: def calculate_average(scores):
      return sum(scores) / len(scores)