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.
['1371', 'u', 'Y', 'W', '1409', 'M', '5621', '1427', '6147', '7131', '6521', '2423', 's', '7617', 'l', 'f', '8851', '2665', 'G', '6831']