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.
['8121', 'z', 'N', 'Z', 'W', 'W', 'Q', '4005', '8835', '3809', 'j', '647', '3875', '8539', '3411', '559', '161', '7141']