A bat flies toward a wall at a speed of 3.5 m/s. As it flies, the bat emits an ultrasonic sound wave with frequency 30.5 kHz. What frequency does the bat hear in the reflected wave?

Question

A bat flies toward a wall at a speed of 3.5 m/s. As it flies, the bat emits an ultrasonic sound wave with frequency 30.5 kHz. What frequency does the bat hear in the reflected wave?
  kHz