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.
['a', '7729', '3657', '4505', '151', 'e', '3047', 'R', 'b', '5289', '4219', 'Z', '6031', '6793', '2017', '3405', 'u', '2897', '5525', '8969']