If in a medium wave's wavelength is decreased, what will happen to its frequency?
Increase
In a Medium the velocity of a sound wave is fixed if the Medium's property are not changed
Also v = λ f
Where v = wave velocity
λ = wavelength
f = wave frequency
So if v is constant and wavelength is decreased you don't need physics to tell you what will happen to its frequency.
v = λ↓f↑
The frequency will have to increase to keep the velocity constant.
Simple maths