$\frac{{{{\rm{V}}_{10}}}}{{2l}} - {\rm{n}} = 3$
$\frac{V_{15}}{V_{10}}=\frac{n+4}{n+3}$ but $V_{t}=V_{0}+0.6 t$
$\frac{332+9}{332+6}=\frac{n+4}{n+3}$
$\mathrm{n}=110 \mathrm{\,Hz}$
(image)
[$A$] The time $\mathrm{T}_{A 0}=\mathrm{T}_{\mathrm{OA}}$
[$B$] The velocities of the two pulses (Pulse $1$ and Pulse $2$) are the same at the midpoint of rope.
[$C$] The wavelength of Pulse $1$ becomes longer when it reaches point $A$.
[$D$] The velocity of any pulse along the rope is independent of its frequency and wavelength.
