A cylindrical tube open at both ends has a fundamental frequency f in air. The tube is dipped vertically in water so that half of it is in water. The fundamental frequency of the air column is now………
$ {f_L \over f_S } = { \nu + \nu_L \over \nu + \nu_S } $ using this equation the frequency of reflected sound heard by the girl, $ f_L = { \nu + \nu_L \over \nu -\nu_S} f_S $