$A$ car covers four successive $3 \, km$ stretches at speeds of $10 \, km/h$,$20 \, km/h$,$30 \, km/h$,and $60 \, km/h$ respectively. Its average speed over this total distance is (in $km/h$):

$A$ 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 $B$?

$A$ and $B$ are $20 \text{ km}$ apart. $A$ can walk at an average speed of $4 \text{ km/hr}$ and $B$ at $6 \text{ km/hr}$. If they start walking towards each other at $7 \text{ a.m.}$,when will they meet? (in $\text{a.m.}$)

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

If a $110 \, m$ long train passes a man walking at a speed of $6 \, km/h$ against it in $6 \, s$,it will pass another man walking at the same speed in the same direction in how many seconds?

$A$ goods train and a passenger train are running on parallel tracks in the same direction. The driver of the goods train observes that the passenger train coming from behind overtakes and crosses his train completely in $60 \, s$. Whereas a passenger on the passenger train marks that he crossed the goods train in $40 \, s$. If the speeds of the trains are in the ratio of $1:2$,find the ratio of their lengths.