input: Write a python function to sum the elements of an array and multiply by a given number.
  assert sum_arr([1, 2, 3, 4, 5], 2) == 30
  assert sum_arr([10, 20, 30], 3) == 180
  assert sum_arr([5, 10, 15], 1) == 30
output: def sum_arr(arr, multiplier):
      return sum(arr) * multiplier