Pratiksha dissolved $105 \mathrm{g}$ of copper sulphate in $250 \mathrm{g}$ of water at $298 \mathrm{K}$. She stirred the solution thoroughly and after filtering it $25 \mathrm{g}$ of copper sulphate was obtained as the residue. In order to dissolve the obtained residue in the above solution at the same temperature, the amount of water that should be added by her will be