A ball is projected vertically upwards at$t=0$. The ball is at same height at time $4 \mathrm{s}$ and $8 \mathrm{s}$. The maximum height attained by the ball is 

$\left(g=10 \mathrm{m} {{\mathrm{s}}^{-}}^2\right)$

The velocity-position graph of a particle moving in a straight line along$x$-axis is given below. Acceleration of particle at$x=2 \mathrm{m}$ is

The acceleration-position graph of a particle moving in a straight line along $x$-axis is shown in figure. The particle starts from rest at origin. The maximum speed of particle is

The acceleration-time graph of a particle moving in a straight line along $x$-axis is given below. If initial velocity of the particle is $4 \mathrm{m} {\mathrm{s}}^{-1}$, then velocity at $t=2 \mathrm{s}$ is

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