When you multiply a wave's (wavelength) times (frequency), the product is always the speed of the wave.
(wavelength) x (frequency) = wave speed
So if you divide the speed by either number, you always get the other number.
speed / wavelength = frequency
speed / frequency = wavelength
AND ... if you divide either of those quantities by some number,
the other one automatically gets multiplied by the same number.
This is a pretty clear example of of an inversely proportional relationship.