A tuning fork of frequency '$n$' is held near the open end of a tube which is dipped in water and length of the tube is adjusted until resonance occurs. If the two shortest lengths that produce resonance are $l_1$ and $l_2$, the speed of sound in air is (neglect end correction)

The ratio of the speed of sound in helium gas to that in nitrogen gas at $295$ K is ( Ratio of specific heats $\gamma$ for helium is $\dfrac{5}{3}$ and that for nitrogen is $\dfrac{7}{5}$. Molecular weight for helium and nitrogen are $4$ and $28$ respectively.)

A train is moving towards a stationary observer with speed $34$ m/s. A train sounds a whistle of frequency $450$ Hz. If the speed of sound is $340$ m/s, the frequency heard by the observer in Hz is

Four electric charges $+q$ , $+q$ , $-q$ , and $-q$ are placed in order at the corners of a square of side '$2r$'. The electric potential at a point midway between the two negative charges is