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.
['H', '8733', '2039', 'a', 'L', 'N', '2285', '1921', '5309', 'i', 'u', '4539', '6013', '5393', '1499', 'Z', 'l', 'H', 'X', 'k']