Two open organ pipes of lengths \( 60 \, \text{cm} \) and \( 90 \, \text{cm} \) resonate at \( 6^{\text{th}} \) and \( 5^{\text{th}} \) harmonics respectively. The difference of frequencies for the given modes is _______ \( \text{Hz} \).
(Velocity of sound in air \( = 333 \, \text{m/s} \))
