input: Write a python function to find the sum of all multiples of 3 and 5 below a given number.
  assert sum_of_multiples_of_3_and_5(10) == 23
  assert sum_of_multiples_of_3_and_5(20) == 78
  assert sum_of_multiples_of_3_and_5(50) == 543
output: def sum_of_multiples_of_3_and_5(n):
      return sum(x for x in range(n) if x % 3 == 0 or x % 5 == 0)