input: Write a python function to find the average of positive numbers in a list.
  assert find_average([1, 2, 3, 4]) == 2.5
  assert find_average([-1, -2, 3, 4]) == 3.5
  assert find_average([10, 20, -30, 40, 50]) == 30.0
output: def find_average(numbers):
      positives = [num for num in numbers if num > 0]
      if not positives:
          return 0
      return sum(positives) / len(positives)