W(K) = 42.1 g/mol
W(O) = 16.00 g/mol

K2O = K(39.1) + O(16.00) = 94.2 g/mol

W(K) = (39.1 / 94.2) 100% = 41.5%
W(O) = (16.00 / 94.2) 100% = 17.0%