A source is oscillating between $A$ and $C$ with its position varying with time as $y=900 \cos \left(\frac{\pi t}{6}\right),$ where $y$ is in metre. An observer is situated at a distance $12 \mathrm{m}$ along the perpendicular bisector of $AC.$ Frequency emitted by source is $300 \mathrm{Hz}.$ Approximate frequency observed by observer when source is at $O$ is $5n \mathrm{Hz}.$ Value of $n,$ is [Velocity of sound in air $=300 \mathrm{m} {\mathrm{s}}^{-1}$]

A rod of mass $1.5 \mathrm{kg}$ and length $40 \mathrm{cm}$ is clamped with the help of two identical strings at the two ends as shown. A block of mass $5 \mathrm{kg}$ has to be connected on the rod. What should be the distance (in $\mathrm{cm}$ ) of point of suspension of block from the first end so that same tuning fork can vibrate the first string in second harmonic and the second string in third harmonic? $\left(g=10 \mathrm{m} {\mathrm{s}}^{-2}\right)$