Waves

Resonance is produced between a turning fork and a resonance column tube with upper end open and lower end closed by water surface. If the frequency of the tuning fork is 800 Hz , and first two resonances are observed at lengths $9.75 \mathrm{~cm}, 31.25 \mathrm{~cm}$,

Find the length at which the third resonance occurs? Also find the speed of sound.