A passenger is standing on the platform at the beginning of the $n^{\mathrm{th}}\left(=3^{\mathrm{rd}}\right)$ coach of a train. The train starts moving with constant acceleration. The third coach passes by the passenger in $△t_1=5.0 \mathrm{s}$ and the rest of the train (including the $3^{\mathrm{rd}}$ coach) in $△t_2=20 \mathrm{s}$. In what time interval $△t$ (in $\sec$) did the last coach passed by the passenger?

A particle has an initial velocity $3\hat{\mathrm{i}}+4\hat{\mathrm{j}}$ and an acceleration of $0.4\hat{\mathrm{i}}+0.3\hat{\mathrm{j}}$. Its speed after $10 \mathrm{s}$ is,

A rocket is moving in a gravity-free space with a constant acceleration of $2{ \mathrm{m} \mathrm{s}}^{-2}$ along $+x$ direction (see figure). The length of a chamber inside the rocket is $4 \mathrm{m}$. A ball is thrown from the left end of the chamber in $+x$ direction with a speed of $0.3{ \mathrm{m} \mathrm{s}}^{-1}$ relative to the rocket. At the same time, another ball is thrown in $-x$ direction with a speed of $0.2{ \mathrm{m} \mathrm{s}}^{-1}$ from its right end relative to the rocket. The time in seconds when the two balls hit each other is