Mixture Capacity in Three Vessels

Let the capacity of each vessel be $V$ litres. Water in P = $\frac{1}{5}V$, Water in Q = $\frac{3}{10}V$, Water in R = 0. Total water in vessel S = $\frac{1}{5}V + \frac{3}{10}V = \frac{1}{2}V$. Total capacity of vessel S = $3V$. Ratio of water in vessel S = $\frac{V/2}{3V} = \frac{1}{6}$. When 30 litres is drawn, water removed = $\frac{1}{6} \times 30 = 5$ litres. Remaining water = $\frac{V}{2} - 5 = 40 \implies \frac{V}{2} = 45 \implies V = 90$ litres.