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 $m$):

$A$ person crosses a $600\, m$ long street in $5\, minutes$. What is his speed 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?

$A$ man walks from $A$ to $B$ and back in a certain time at the rate of $3.5 \text{ km/h}$. If he had walked from $A$ to $B$ at the rate of $3 \text{ km/h}$ and back from $B$ to $A$ at the rate of $4 \text{ km/h}$,he would have taken $5 \text{ minutes}$ longer. Find the distance between $A$ and $B$ (in $\text{km}$).

$A$ person goes from point $A$ to $B$ with a speed of $55 \text{ km/hr}$ and returns with a speed of $65 \text{ km/hr}$. What is the average speed (in $\text{km/hr}$) of the person while going and coming back from $A$ to $B$?

$A$ car moving in the same direction passes a man walking at $4\, km/h$. The man can see the car for $3\, minutes$ and the visibility is up to a distance of $130\, m$. The speed of the car is: (in $km/h$)