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An automobile travels from city $A$ to city $B$ and returns to city $A$ by the same route. The speed of the vehicle during the onward and return journeys were constant at $60km/h$ and $90 km/h$, respectively. What is the average speed in $km/h$ for the entire journey?

1. $72\hspace{0.2cm} km/h$
2. $73\hspace{0.2cm} km/h$
3. $74\hspace{0.2cm} km/h$
4. $75 \hspace{0.2cm}km/h$

75km/hr

Can u plz chk the answer key?

$\text{Average Speed} = \dfrac{\text{Total Distance travelled}}{\text{Total time taken}}$

Total Distance travelled $=2d$

Total time taken $=$ Time taken from $A$ to $B$ + Time taken from $B$ to $A$

$=\dfrac{d}{60}+\dfrac{d}{90}$

Average speed $=\dfrac{2d}{\dfrac{d}{60}+\dfrac{d}{90}}=\dfrac{2d*180}{5d}=72km/hr$

Hence, option (A) is correct

$average speed $$=$$ total \ distance / total\ time$

$= (d + d) / (d/60 + d/90)$

$= (2 * 60 *90) / (60 +90)$

$=72 km/hr$

### 1 comment

how total distance 60*90??

that is speed ,na?