A bat glides along the positive x - axis (to the right) at a uniform velocity of 16.0 m/s while a rodent, which is to the right of the bat by a fair distance, runs towards the bat (in the leftward direction) at a uniform velocity of 4.00 m/s. The bat emits a sound wave at a frequency of 30.0 x 103 Hz. The wave will strike the rodent and perfectly reflect back towards the bat. What is the frequency of this echo as detected by the bat