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.
['P', '521', '5667', '125', '9875', 'u', 'L', 'n', 'G', '4071', '8201', '4009', 'h', '8881', '1521', 'U', 'f', '7719']