$K E=\frac{1}{2} m V^2$
$\Rightarrow V=\sqrt{\frac{2 K E}{m}}$
$=v \cos 60^{\circ} \times \frac{2 \pi m}{e B}$
$=\sqrt{\frac{2 \times 2 \times 1.6 \times 10^{-19}}{1.6 \times 10^{-27}}} \times \cos 60^{\circ} \times \frac{2 \pi \times 1.6 \times 10^{-27}}{1.6 \times 10^{-19} \times \frac{\pi}{2} \times 10^{-3}}$
$=2 \times 10^4 \times \frac{1}{2} \times 4 \times 10^{-5}$
$=4 \times 10^{-1} m =40\,cm$
Reason : Magnetic field energy density is proportional to square of current.

$[1]$ The magnetic field strength may have been increased while the particle was travelling in air
$[2]$ The particle lost energy by ionising the air
$[3]$ The particle lost charge by ionising the air
If $\frac{x_0}{x_1}=3$, the value of $\frac{R_1}{R_2}$ is.