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.
['3851', 'M', 'O', 'W', 'X', '3521', 'o', 'p', 'O', 'k', '4835', 'j', 'B', 'n', 'v', 'S', '7151', 'S', '1767', 'f']