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.
['6949', '5141', 'I', '8541', '1881', '8993', 't', 't', 'U', 'n', '1203', 'J', 'P', '3325', 'g', 'R', '7337', '7009', '7901']