$A$ person travels from $P$ to $Q$ at a speed of $40 \, km/hr$ and returns by increasing his speed by $50 \%$. What is his average speed for both the trips? (in $km/hr$)

Walking at $3 \, km/h$,Pintu reaches his school $5 \, minutes$ late. If he walks at $4 \, km/h$,he will be $5 \, minutes$ early. The distance of Pintu's school from his house is (in $km$):

Two trains,each of length $90\, m,$ run on parallel tracks. When running in the same direction,the faster train passes the slower train completely in $18\, s,$ but when they are running in opposite directions approaching each other at the same speeds as before,they cross each other in $9\, s.$ Find the speed of each train (in $m/s$).

Two places $P$ and $Q$ are $162 \, km$ apart. $A$ train leaves $P$ for $Q$ and simultaneously another train leaves $Q$ for $P$. They meet at the end of $6 \, hours$. If the former train travels $8 \, km/h$ faster than the other,then the speed of the train from $Q$ is (in $km/h$):

$A$ man walking at $3 \, km/h$ crosses a square field diagonally in $2 \, minutes$. The area of the field (in $m^2$) is:

$A$ car covers the first $39 \, km$ of its journey in $45 \, minutes$ and the remaining $25 \, km$ in $35 \, minutes.$ What is the average speed of the car (in $km/h$)?