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.
['1897', '3951', '461', '3955', 'Z', 'U', 'A', '4083', '3715', 'u', 'S', 'u', '5083', '1531', 'T', '6661', 'x', 'U', 'J', '1559']