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.
['3859', '1389', 'q', '7659', '8745', 'D', '2907', '5767', 'N', '8565', 'N', 'R', 'S', 'Y', 'P', '8057', 'a']