A train starting from rest first accelerates uniformly up to a speed of $80 \mathrm{~km} / \mathrm{h}$ for time $t$, then it moves with a constant speed for time 3t. The average speed of the train for this duration of journey will be (in $\mathrm{km} / \mathrm{h}$ ) :

A man walks on a straight road from his home to a market $2.5\; km$ away with a speed of $5 \;km h ^{-1} .$ Finding the market closed, he instantly turns and walks back home with a speed of $7.5 \;km h ^{-1} .$ What is the average speed (in $km/h$) of the man over the interval of time $0$ to $50\; min$ ?

A car is moving along a straight line, say $OP$ in given figure. It moves from $O$ to $P$ in $18\; s$ and returns from $P$ to $\mathrm{Q}$ in $6.0\; s$. What are the average velocity and average speed of the car in going from $O$ to $P$ and back to $Q?$

A boy walks to his school at a distance of $6\, km$ with constant speed of $2.5\, km/hour $ and walks back with a constant speed of $4 \,km/hr.$ His average speed for round trip expressed in $km/hour$, is

A car moves for half of its time at $80\, km/h$ and for rest half of time at $40\, km/h$. Total distance covered is $60 \,km$. What is the average speed of the car.........$km/h$

A car travels half the distance with constant velocity of $40\, kmph$ and the remaining half with a constant velocity of $60 \,kmph$. The average velocity of the car in $kmph$ is