**Category:** Wave Physics  
**Subtype:** Wave Properties (Frequency and Wavelength)  
**Problem Description:** This problem involves calculating the frequency of a wave based on its wavelength and the relationship between displacement and velocity of particles in the wave. 

**Key Information:**
- Wavelength (λ) = 40 cm
- Distance between points M and N (MN) = 90 cm
- Displacement at point M = 2 cm
- Velocity at point N = 125.6 cm/s

**Objective:** Find the frequency of the wave.