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.
['3791', 'W', 'k', 'p', 'Z', 'R', 'o', 'X', '6323', 'u', 'n', '6539', '7995', '9305', 'b', '27', '8207', 'z']