Refer to figure.
Radio waves are electromagnetic waves that travel outwards from a transmitter at the speed of light. They are transverse waves, meaning that the electric and magnetic fields involved travel from side to side (or up and down) whilst the wave goes "forwards".

There are a few components of these waves that you are required to know. Two of the most important are wavelength and frequency.

We have already mentioned that the wave travels at the speed of light, but that does not tell us how long the wave is. The wavelength is the distance from one part of a wave cycle to the same part of the next wave cycle. For instance, from one trough to the next, or from one peak to the next.

We can use this information to tell us how many times a wave peak (or trough) will pass us every second, which is the frequency of the wave. This can also be described as the number of wave cycles per second, measured in Hertz (Hz)

The relationship between these values is by the formula:

c = f x λ

c = the speed of light (m/s)
f = frequency (Hz)
λ (Lambda) = wavelength (m)

The speed of light (c) is 300 000 000 m/s, so 3x10⁸ m/s. The frequency is 400 kHz, which is 400 x10³ Hz or 400 000 MHz in long form. Remember to use metric units, Metres, Hertz and Metres per Second only.

We can rearrange the formula to be:
λ = c / f

and then plug in the information to give:
λ = 3x10⁸ / 400x10³ = 750 m