$\dfrac{V_\text{air}}{V_\text{tank}} = \left( \dfrac{\frac{1}{3}h}{h} \right)^3$
$\dfrac{V_\text{tank} - V_\text{water}}{V_\text{tank}} = \dfrac{1}{27}$
$27V_\text{tank} - 27V_\text{water} = V_\text{tank}$
$27V_\text{water} = 26V_\text{tank}$
$\dfrac{V_\text{water}}{V_\text{tank}} = \dfrac{26}{27}$ ← answer
