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.
['n', '805', '7509', 'z', '7681', '4519', 'k', '1621', 'M', '8853', '7961', 'u', 'E', 'o', 'W', 'Q', 'U', '1275', '6915', '6859']