(Velocity of sound in air is $340\,ms ^{-1}$ )
$\lambda=\frac{ V }{ f }=\frac{340}{340}=1\,m$
First resonating length $=\frac{\lambda}{4}=25\,cm$
Second resonating length $=\frac{3 \lambda}{4}=75\,cm$
Third resonating length $=\frac{5 \lambda}{4}=125\,cm$
Height of water required $=125-75=50\,cm$

