A balloon is moving upwards with velocity $10 \mathrm{m} {\mathrm{s}}^{-1}$. It releases a stone which comes down to the ground in $11 \mathrm{s}$. The height of the balloon from the ground at the moment when the stone was dropped is: 

Figure shows the position of a particle moving on $X$ -axis as a function of time. 

Two parallel rail tracks run North-South. Train $A$ moves due North with a speed of $54 \mathrm{km} {\mathrm{h}}^{-1}$ and train $B$ moves due South with a speed of $90 \mathrm{km} {\mathrm{h}}^{-1}$. A monkey runs on the roof of the train $A$ with a velocity of $18 \mathrm{km} {\mathrm{h}}^{-1}$ with respect to the train $A$, in a direction opposite to that of $A$. Calculate
$\left(\mathrm{a}\right)$ the relative velocity of $B$ with respect to $A$.
$\left(\mathrm{b}\right)$ the relative velocity of the ground with respect to $B$.
$\left(\mathrm{c}\right)$ the velocity of a monkey as observed by a man, standing on the ground.
$\left(\mathrm{d}\right)$ the velocity of the monkey as observed by a passenger of the train $B$.

An object $A$ is moving with $10 \mathrm{m} {\mathrm{s}}^{-1}$ and $B$ is moving with $5 \mathrm{m} {\mathrm{s}}^{-1}$, in the same direction of the positive $x$-axis. $A$ is $100 \mathrm{m}$ behind $B$, as shown in the figure. Find the time taken by $A$ to meet $B$.