Two cars are moving in the same direction, with the same speed of $30 \mathrm{km} {\mathrm{h}}^{-1}$. They are separated by $5 \mathrm{km}$. What is the speed of the car moving in the opposite direction, if it meets the two cars at an interval of $4 \mathrm{minutes}$?

Three elephants, $A$, $B$ and $C$, are moving along a straight line with constant speed, in the same direction, as shown in the figure. Speed of $A$ is $5 \mathrm{m} {\mathrm{s}}^{-1}$ and speed of $C$ is $10 \mathrm{m} {\mathrm{s}}^{-1}$. Initially, the separation between $A$ and $B$ is $d$ and between $B$ and $C$ is also $d$. When $B$ catches $C$, the separation between $A$ and $C$ becomes $3d$. Then, the speed of $B$ will be