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.
['B', 't', 'N', 'Z', 'q', '4121', 'N', 't', 'p', 'W', '2669', 't', '6507', '4869', '6529', '5841', '3669', '1787', 'A', '7291']