Mass per unit length, $\mu=0.001 \mathrm{kg} / \mathrm{m}$
Frequency, $v=\frac{1}{2 l} \sqrt{\frac{T}{\mu}}$
${v_{1}=\frac{1}{2 \times 0.516} \sqrt{\frac{20}{0.001}}}$
${v_{2}=\frac{1}{2 \times 0.491} \sqrt{\frac{20}{0.001}}}$
$\therefore \quad$ Number of beats $=v_{1}-v_{2}=7$
| $(A)$ Temperature of gas is made $4$ times and pressure $2$ times | $(P)$ speed becomes $2\sqrt 2$ times |
| $(B)$ Only pressure is made $4$ times without change in temperature | $(Q)$ speed become $2$ times |
| $(C)$ Only temperature is changed to $4$ times | $(R)$ speed remains unchanged |
| $(D)$ Molecular mass of the gas is made $4$ times | $(S)$ speed remains half |

$y_{1}=5 \sin 2 \pi(x-v t) \,c m\,$
$y_{2}=3 \sin 2 \pi(x-v t+1.5) \,c m$
These waves are simultaneously passing through a string. The amplitude of the resulting wave is.........$cm$