$n = \frac{1}{{2l}}\sqrt {\frac{T}{{\pi {r^2}\rho }}} = \frac{1}{l}\sqrt {\frac{T}{{\pi {d^2}\rho }}} $ ($d =$ Diameter of string)
==> $\frac{{{n_1}}}{{{n_2}}} = \frac{{{l_2}}}{{{l_1}}}\sqrt {\frac{{{T_1}}}{{{T_2}}} \times {{\left( {\frac{{{d_2}}}{{{d_1}}}} \right)}^2} \times \left( {\frac{{{\rho _2}}}{{{\rho _1}}}} \right)} $
$ = \frac{{35}}{{36}}\sqrt {\frac{8}{1} \times {{\left( {\frac{1}{4}} \right)}^2} \times \frac{2}{1}} = \frac{{35}}{{36}}$$ \Rightarrow {n_2} = \frac{{36}}{{35}} \times 360 = 370$
Hence beat frequency = ${n_2} - {n_1} = 10$
(Useful information) : $\sqrt{167 R T}=640 j^{1 / 2} mole ^{-1 / 2} ; \sqrt{140 RT }=590 j ^{1 / 2} mole ^{-1 / 2}$. The molar masses $M$ in grams are given in the options. Take the value of $\sqrt{\frac{10}{ M }}$ for each gas as given there.)

where $x$ and $y$ are in metres and $t$ is in seconds. The ratio of maximum particle velocity to the wave velocity is
$(A)$ With a node at $O$, the minimum frequency of vibration of the composite string is $v_0$
$(B)$ With an antinode at $O$, the minimum frequency of vibration of the composite string is $2 v_0$
$(C)$ When the composite string vibrates at the minimum frequency with a node at $O$, it has $6$ nodes, including the end nodes
$(D)$ No vibrational mode with an antinode at $O$ is possible for the composite string