input: Write a function to calculate the average temperature from a list of temperatures.
  assert average([70, 75, 80, 85, 90]) == 80.0
  assert average([60, 65, 70, 75, 80, 85, 90]) == 75.0
  assert average([100, 95, 90, 85, 80]) == 90.0
output: def average(temperatures):
      return sum(temperatures) / len(temperatures)