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.
['929', '6833', 'h', '6677', '6939', '4761', '7499', 'Y', '3507', '5543', '3919', 'b', '7475', '3711', '9507', '4509', '7431', '243', 'Z', 'l']