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.
['2705', '7625', 'O', 'S', 'f', '9489', 'G', '6549', '1797', 'L', 't', 'N', 'U', 'w', '6145', 'R', '3715', 'H', 'w', '5481']