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.
['x', 'M', '4087', 'A', '463', 'T', '4249', '8805', 'K', '4245', '6345', 'C', 'X', '9353', '6403', '4789', '7867', 'R', 't']