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.
['4989', 'x', 'f', '6685', 'a', 'r', '3721', 'Q', '709', 'z', 'p', '8979', 'L', 'L', 'm', 'l', '7109', '5985', 'U', '2193']