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.
['4283', 'r', 'M', 'C', '75', 'P', '2945', 'H', '2001', 'b', '131', 'y', '5271', 'G', 'R', 'L', 'R', 'b']