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.
['3497', '761', '6157', 'J', 'n', '139', '9895', '2939', 'K', 'I', '2161', 'c', 'a', '485', '7959', 'i', '4753', '3305', 'G', '3475']