In this task, you are given an input list A. You need to find all the elements of the list that are numbers and calculate their sum.
['K', '5815', 'E', 'c', '7379', '4995', 'w', 'B', 'j', '6227', 'B', '733', '1491', '3191', '9413', '2751', 'O', 'J', '6239', '3521']