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 \( 20.0 \, \text{cm} \) for a tuning fork of frequency \( 400 \, \text{Hz} \) is used. The velocity of the sound at room temperature is \( 336 \, \text{m s}^{-1} \). The third resonance is observed when the air column has a length of _______ \( \text{cm} \).
