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.
['4487', '4103', 'U', 'f', '5839', '9075', 'G', '3099', '5281', '2393', '8781', '7375', 'D', '9125', 'S', 'Y', '9151', 'n', 't', '8629']