Effective molality = 0.6 + 1.6 + 0.4 = 2.6m
For same boiling point, the molality of another solution should also be 2.6 m. Now, 18.1 weight percent solution means 18.1 gm solute is present in 100 gm solution and hence, (100 – 18.1 =) 81.9 gm water.
∴ Molar mass of solute, M = 85