A ball is projected vertically upwards. If it covers equal distances in $4^{\text{th }}$ and $7^{\text{th }}$ second, then velocity of projection is $\left(g=10 \mathrm{m} {\mathrm{s}}^{-2}\right)$

A boat goes from point  $A$ to $B$ downstream of a river flowing with speed  $6 \mathrm{km} {\mathrm{h}}^{-1}$ and then comes back to point $A$. If time taken in upstream journey is double of that in downstream journey then speed of boat in still water is

Graph of  $\left(\frac{1}{V}\right)$ versus $x$ for a particle under motion is shown in figure, where $v$ is velocity and $x$ is position. Find the time taken by particle to move from $x=4 \mathrm{m}$ to $x=12 \mathrm{m}$.