In an experiment to determine the velocity of sound in air at room temperature using a resonance tube, the first resonance is observed when the air column has a length of 20.0 cm for a tuning fork of frequency 400 Hz is used. The velocity of the sound at room temperature is $336 \mathrm{~m~s}^{-1}$. The third resonance is observed when the air column has a length of $\_\_\_\_$ cm.