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.
['G', 'C', 'X', 'R', 'X', '3935', 'q', '2571', 'G', '8899', '2881', '1629', '7239', '2129', 'b', 'X', 'B', '3303', 'o']