$ = \,\,\frac{X}{{X\,\, + \,\,\frac{{1000}}{{78}}}}\,\, = \,\,\frac{X}{{X\,\, + \,\,12.82}}$
$ \Rightarrow \,\,0.2\,\, = \,\,\,\frac{X}{{X\,\, + \,\,12.82}}$ $ \Rightarrow \,\,X\,\, = \,\,3.2$
$[K_f \,(H_2O)= 1.86\,K\,kg\,mol^{-1}$ ; $K_b\,(H_2O) = 0.52\,K\,kg\,mol^{-1} ]$