$ \Rightarrow \,\,\,\frac{{640\,\, - \,\,600}}{{600}}\,\, = \,\,\frac{{2.175\,\, \times \,\,78}}{{{{(Mol.wt.)}_{solute}}\,\, \times \,\,39}}$

$ \Rightarrow \,\,\frac{{40}}{{600}}\,\, = \,\,\frac{{2.175\,\, \times \,2}}{{{{(Mol.wt.)}_{solute}}}}$

$ \Rightarrow \,\,{(M.wt.)_{solute}}\, = \,\,65.25\,\, \approx \,\,65.3$