Explanation

(b)
$\frac{p - p_{s}}{p}$=$\frac{w_{1}M_{2}}{w_{2}M_{1}}$
To produce same lowering of vapour pressure, $\frac{p - p_{s}}{p}$ will be same for both cases.
So, $\frac{W_{(Glucose)} \times 18}{50 \times 180}$=$\frac{W_{(urea)} \times 18}{50 \times 60}$
$\ \ \ \ \ \ \ \ \ \ \ \ \ W_{(Glucose)}$=weight of glucose
$W_{(urea)}$ =weight of urea
or $\ \ \ \frac{W_{(Glucose)} \times 18}{50 \times 180} = \frac{1 \times 18}{50 \times 60}$
$\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ {\ \ \ W}_{(urea)}$=3