Question : A man drives his car for 24 km at a speed of 48 km/hr and for the next 1.5 hours, he drives at a speed of 80 km/hr. Find his average speed (in km/hr) for the entire journey.
Option 1: 64
Option 2: 72
Option 3: 68
Option 4: 75
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Correct Answer: 72
Solution :
Average speed = $\frac{\text{Total distance}}{\text{Total time}}$
Time taken to travel the distance of 24 km with a speed of 48 km/hr
$ = \frac{24}{48}= \frac{1}{2}= 0.5$ hr
The man drives for the next 1.5 hours he drives at a speed of 80 km/hr
∴ Distance travelled in 1.5 hours with a speed of 80 km/hr
$= 80\times 1.5 = 120$ km
∴ Total distance travelled
= $24$ km + $120$ km = $144$ km
And the total time taken
= $0.5$ hour + $1.5$ hour = $2$ hour
$\therefore $ Average speed = $\frac{144}{2} = 72$ km/hr
Hence, the correct answer is 72.
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