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.
['6921', 'a', 'U', '2821', '2111', 'h', '3937', '3697', 'd', '6309', 'q', '7961', '5287', '5375', '2721', '4523', 'V', '8633', 'c', '7643']