W (k) = 39,7%

w (Mn) = 27,9%

w (0) - остальное

формула - ?

W (O) = 100% - 39.7% - 27.9% = 32.4%

32.4% = 0.324

39.7% = 0.397

27.9% = 0.279

n (K) = 0.397 / 39 = 0.01018

n (Mn) = 0.279 / 55 = 0.00507 моль

n (O) = 0.324 / 16 = 0.02025 моль

n (K) : n (Mn) : n (O) = 2 : 1 : 4 = > K2MnO4

Ответ: K2MnO4