Initial conc. $1\, m\quad \quad \quad \quad \quad \quad 0\quad \quad \quad \quad 0$

Final conc. $(1-0.4) \,m \quad \quad 4 \times 0.4 \quad \quad 0.4\, m$

$\quad \quad \quad \quad \quad \quad =0.6\,m \quad \quad \quad =1.6\,m$

Effective molality $=0.6+1.6+0.4=2.6 \,m$

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.

$\text { Now, } \quad 2.6=\frac{18.1 / M }{81.9 / 1000}$

$\therefore$ Molar mass of solute, $M =85$