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.
['k', 'Q', '5775', '389', '129', '3957', 't', '3433', '8937', '6537', '3069', '4429', '6645', 'f', '1667', 'E', 'R', 'x']