During the first $18\,min$ of a $60\,min$ trip, a car has an average speed of $11\,m / s$. What should be the average speed for remaining $42\,min$ so that car is having an average speed of $21\,m / s$ for the entire trip?

A $150 \,m$ long train is moving with a uniform velocity of $45 \,km/h$. The time taken by the train to cross a bridge of length $850 $ meters is..........$sec$

A person travels $x$ distance with velocity $v_1$ and then $x$ distance with velocity $v_2$ in the same direction. The average velocity of the person is $v$, then the relation between $v , v _1$ and $v _2$ will be :

A person goes from point $P$ to point $Q$ covering $1 / 3$ of the distance with speed $10 \,km / h$, the next $1 / 3$ of the distance at $20 \,km / h$ and the last $1 / 3$ of the distance at $60 \,km / h$. The average speed of the person is ............ $km / h$

A point traversed $3 / 4$ th of the circle of radius $R$ in time $t$. The magnitude of the average velocity of the particle in this time interval is

A car moves from $X$ to $Y$ with a uniform speed $v_1$ and returns to $Y$ with a uniform speed $v_2$ . The average speed for this round trip is