The distance between Delhi and Ambala is $200 \mathrm{k}\mathrm{m}$. A train travels the first $100 \mathrm{k}\mathrm{m}$ at a speed of $50 \mathrm{k}\mathrm{m}{\mathrm{h}}^{-1}$. How fast must the train travel the next $100 \mathrm{k}\mathrm{m}$, so as to average $70 \mathrm{k}\mathrm{m}{\mathrm{h}}^{-1}$ for the whole journey?

A body goes from $A$ to $B$ with a speed of $60 {\mathrm{kmh}}^{-1}$, and then returns to the starting point with a speed of $40 {\mathrm{kmh}}^{-1}$. Find the average velocity of the body.

A car is moving along a straight line $\mathrm{OP}$ as shown below. It moves from $\mathrm{O}$ to $\mathrm{P}$ in $18 \mathrm{s}$ and returns from $\mathrm{P}$ to $\mathrm{Q}$ in $6 \mathrm{s}$. What are the average velocity and average speed of the car in going from to $\mathrm{O}$ to $\mathrm{P}$?

A car is moving along a straight line $\mathrm{OP}$ as shown below. It moves from $\mathrm{O}$ to $\mathrm{P}$ in $18 \mathrm{s}$ and returns from $\mathrm{P}$ to $\mathrm{Q}$ in $6 \mathrm{s}$. What are the average velocity and average speed of the car in going from to $\mathrm{O}$ to $\mathrm{P}$ and back to $Q?$