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.
['5431', 'X', '2951', 'w', '653', 'u', 'l', 'Z', 'W', '8661', '9235', 'P', 'R', 'S', '9789', '1607', 'M', '5497', 't', 'm']