First convert the 125 g of CuSO4 to moles using the molar mass of CuSO4. Then use the mole ratio of 4 mol O for every 1 mol CuSO4 to get moles of O. Then use the molar mass of O to convert from moles of O to grams of O.
125 g CuSO4 • 1 mol CuSO4 / 159.609 g CuSO4 • 4 mol O / 1 mol CuSO4 • 16.0 g O / 1 mol O = 50.1 g oxygen
