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.
['305', 'u', '1857', '5861', '6637', '3353', '4369', '6493', 'G', 'S', '5837', 'Z', '2857', '1623', 'V', '1567', '5205', '6753', 'L', 'j']