A chemistry student weighs out 0.700 kg of an unknown solid compound X and adds it to 3.00 L of distilled water at 18C. After 10 minutes of stirring, only some of the X has dissolved. The student drains off the solution, then washes, dries and weighs the X that did not dissolve. It weighs 0.280 kg. Calculate the solubility of X in water at 18C.