$A$ boy goes to his school from his house at a speed of $3 \text{ km/hr}$. If he takes $5 \text{ hours}$ in total for going and coming back,the distance between his house and school is (in $\text{km}$):

Two trains are running at a speed of $40 \, km/h$ and $20 \, km/h$ respectively in the same direction. The fast train completely passes a man sitting in the slow train in $5 \, seconds$. The length of the fast train is (in $m$):

Two places $A$ and $B$ are $100 \, km$ apart on a highway. One car starts from $A$ and another from $B$ at the same time. If the cars travel in the same direction at a constant speed,they meet in $5 \, hours$. If the cars travel towards each other,they meet in $1 \, hour$. What is the speed of the car running faster? (in $km/h$)

Two friends started for a place,one by motorcycle and the other by train. The speed of the motorcycle is $30 \, km/h$ and that of the train is $24 \, km/h$. The first one takes $6 \, hr \, 12 \, min$ to reach the destination. Find the time taken by the second one to reach the destination (in $hr$).

$A$ man covers a certain distance by car driving at $70 \, km/hr$ and he returns to the starting point riding on a scooter at $55 \, km/hr$. Find his average speed for the whole journey in $km/hr$.

Two trains of equal length take $10\, seconds$ and $15\, seconds$ respectively to cross a telegraph post. If the length of each train is $120\, metres,$ in what time (in $seconds$) will they cross each other when travelling in opposite directions?