Two waves of equal frequency f and velocity v travel in opposite directions along the same path. The waves have amplitudes A and 3A. Then

$\mathrm{y}={\mathrm{y}}_1+{\mathrm{y}}_2$

$=\mathrm{A} \sin (\mathrm{\omega t}-\mathrm{kx})+3\mathrm{A} \sin (\mathrm{\omega t}+\mathrm{kx})$

$=\mathrm{A} \sin \mathrm{\omega t} \cos \mathrm{kx}-\mathrm{A} \cos \mathrm{\omega t} \sin \mathrm{kx}+3\mathrm{A} \sin \mathrm{\omega t} \cos \mathrm{kx}+3\mathrm{A} \cos \mathrm{\omega t} \sin \mathrm{kx}$

$=4\mathrm{A} \sin \mathrm{\omega t} \cos \mathrm{kx}+2\mathrm{A} \cos \mathrm{\omega t} \sin \mathrm{kx}$

$=2\mathrm{A} \sin \mathrm{\omega t} \cos \mathrm{kx}+2\mathrm{A} \sin (\mathrm{\omega t}+\mathrm{kx})$

It is combination of a stationary and travelling wave.

Maximum amplitude = 4 A.

Minimum amplitude = 2 A

Distance between points having amplitude 4A and 2A will be $=\mathrm{\lambda }/4=\mathrm{v}/4\mathrm{f}$