input: Write a python function to find the sum of all numbers below a given number that are divisible by 3 or 5.
  assert sum_divisible_by_3_or_5(10)==23
  assert sum_divisible_by_3_or_5(100)==2318
  assert sum_divisible_by_3_or_5(1000)==233168
output: def sum_divisible_by_3_or_5(n):
      return sum(x for x in range(n) if x % 3 == 0 or x % 5 == 0)