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.
['7645', 'n', '8403', 'K', 'W', 'f', '6141', 'D', 'v', 'U', 'I', 'X', 'R', '6261', 'F', '5251', 'L', 'I', 'k', 'X']