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.
['t', '5445', '8847', '6421', 't', '157', 'D', '6083', 'L', '187', 'z', 'i', '6547', '2161', '6331', '3343', '7283', '5691', 'k']