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.
['S', 'r', '9143', 'X', '4789', 'x', 'e', 'Z', 't', '4007', '8539', 'l', 'I', '4371', 'x', 'b', 'v', 'z']