А.

Mr(Ca(NO3)2) = 40 + 14×2 + 16×6 = 164

W(Ca) = 40 : 164 × 100% = 24,39 %

W(N) = 14*2 : 164 × 100% = 17,07 %

W(O) = 100 - (24,4 + 17,1) = 58,54 %

Б.

Mr(MnO2) = 55 + 16×2 = 87

W(Mn) = 55 : 87 × 100% = 63,22 %

W(O) = 100 - 63,22 = 36,78 %

В.

Mr(MgSO4) = 24 + 32 + 16×4 = 120

W(Mg) = 24 : 120 × 100% = 20 %

W(S) = 32 : 124 × 100% = 26,67 %

W(O) = 100 - (20 + 26,67) = 53,33 %