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.
['z', '735', '6331', 'b', '5141', 'Q', 'W', 'E', 'F', '2279', '5117', '7969', 's', '5745', 'g', '6457', 'r', 'r', '3999']