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', '4231', 'W', 'P', 'r', 'H', 'X', 't', 'L', 'C', 'J', 'E', 'H', 'H', '6429', 'r', '7241', 'q', '8413']