Mr(CuSO4) = Ar(Cu)+Ar(S)+4Ar(O)
Mr(CuSO4) = 64 + 32 + 4*16 = 160
w(Cu) = Ar(Cu) / Mr(CuSO4) * 100%
w(Cu) = 64 / 160 * 100% = 40%
w(S) = Ar(S) / Mr(CuSO4) * 100%
w(S) = 32 / 160 * 100% = 20%
w(O) = 4Ar(O) / Mr(CuSO4) * 100%
w(O) = 64 / 160 * 100% = 40%