A mixture of MgSO4.7H2O and CuSO4.5H2O is heated at 120°C until a mixture of the anhydrous compounds is produced. If 5.00 g of the mixture gave 3.00 g of the anhydrous compounds, calculate the percentage by mass of MgSO4.7H2O in the mixture. I HAVE BEEN STUCK ON THIS FOR HOURS!! I got a final answer but it's wrong, as it's meant to be 26.6%. ANY HELP WILL BE APPRECIATED.