The frequency separating audible waves and ultrasonic waves is considered to be 20 kHz. What wavelength in air?

The wave equation c = λ x f hold also for the sound, where c is the speed of sound, f is the frequency and λ is the wavelength. So, λ = c / f = 340 m/s / 20,000 /s = 0.017 m = 17 mm.