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.
['Z', '5297', '83', '7725', '8861', 'Q', 'w', '1385', 'u', '6455', 'S', '7359', '6311', '3849', 'L', '3879', '3775', '9757', 'u', '8309']