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.
['3759', '4803', 'n', 's', '8431', '1761', 'V', '1557', 'l', 'l', 'o', 'A', 'd', '6649', '5215', '4925', '7827', '5393', '7771']