Mass of urea = 10g
Mass of water = 90g
Moles of urea =$\large\frac{10g}{60gmol^{-1}}=\frac{1}{6}$
Moles of water =$\large\frac{90}{18}$$=5$
Mole fraction of water
$x_A=\large\frac{5}{5+26}=\frac{5}{31}$
Vapour pressure of solution
$P=P_A^{\large\circ}.x_A$
$\;\;\;=55.3\times \large\frac{5}{31}$
P=53.52 mm Hg