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.
['j', '3779', 'X', '7775', '2397', '7713', 'd', '915', '5147', '2435', '2783', 'O', '3783', 'Q', '5543', 'k', 'f', 'y', '8775']