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.
['171', '7101', '3935', 'X', '7451', '1313', '2157', '6021', 'u', 'w', '6285', '7519', '5607', 'V', '3791', 'H', '2769', 'f']