input: Write a python function to calculate the sum of all numbers less than n that are multiples of 3 or 5.
  assert calculate_sum(10) == 23
  assert calculate_sum(15) == 45
  assert calculate_sum(20) == 78
output: def calculate_sum(n):
      return sum(i for i in range(n) if i % 3 == 0 or i % 5 == 0)