Im doing the test now and i think it's 140. I dont really know how to use the chart and dont know if im correct or not, but Im assuming the "grams of solute per 100g H2O" means that at whatever corresponding point between the temperature and chemical line on the graph has 100g H2O. So if you find 90 degrees on the graph and go up to the NH4Cl line, your Y Axis will say 70. But since the "grams of solute per 100g H2O" this means to satisfy the requirements of having 200g of water, you must double the 70 to 140. (This is my best guess, just as lost as you but im trying)