Also path difference $(\Delta x)$ between the waves at the point of observation is $AP - BP = 25cm$.
Hence
==> $\Delta \phi = \frac{{2\pi }}{\lambda }(\Delta x) = \frac{{2\pi }}{1} \times \left( {\frac{{25}}{{100}}} \right) = \frac{\pi }{2}$
==> $ A = \sqrt {{{({a_1})}^2} + {{({a_2})}^2}} = \sqrt {{{(0.3)}^2} + {{(0.4)}^2}} = 0.5 mm$


$(A)$ The speed of sound determined from this experiment is $332 m s ^{-1}$
$(B)$ The end correction in this experiment is $0.9 cm$
$(C)$ The wavelength of the sound wave is $66.4 cm$
$(D)$ The resonance at $50.7 cm$ corresponds to the fundamental harmonic