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.
['3291', '293', '2225', 'm', '1931', '1881', 'Z', 's', 'r', 'Q', 'z', 'r', 'p', '7879', 'f', '827', 'P', '1331']