What is the frequency of a sound wave in air at 20°C that has a wavelength of 0.4 m?

Answer 1

distance = speed time time
so
wavelength = speed of sound * period T
but
f = 1/T
so
wavelength = speed of sound / f

f = speed /wavelength
look up speed of sound at 20 C in your book (different tables available)

Answer 2

857.5hz?