Answer

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Hint: The proper approach of the solution is first finding the distance travelled by the car.

Then we can use the formula i.e. distance= speed * time

Complete step-by-step answer:

We first need to find the distance travelled by the car in 2 hours with speed 60 km/h.

Distance= Speed X Time …(i)

Where, speed is equal to the speed maintained by the car throughout the journey and time is equal to the time taken by the car to complete the journey.

Therefore, distance= $60km/h\times 2h=120km$

Now we need to calculate the time with which the calculated distance will be covered with a speed of 80 km/h.

Equation (i) can also be written as $Time=\dfrac{Dis\tan ce}{Speed}$

Distance we have is 120 km and the given speed is 80 km/h.

Therefore, $Time=\dfrac{120km}{80km/h}$

On simplifying we have,

$Time=\dfrac{3}{2}h=1.5h$

Hence, the answer is 1.5 hours.

Note: We must remember that the speed must be the same throughout the distance travelled. Hence, the speed must be uniform and if not the formula distance=speed x time cannot be used.

In the question it is not exactly mentioned that the speed is uniform throughout the journey and the distance travelled at first must be used to calculate the time when speed increases to 80 km/h. We must be intuitive and understand these things.

Then we can use the formula i.e. distance= speed * time

Complete step-by-step answer:

We first need to find the distance travelled by the car in 2 hours with speed 60 km/h.

Distance= Speed X Time …(i)

Where, speed is equal to the speed maintained by the car throughout the journey and time is equal to the time taken by the car to complete the journey.

Therefore, distance= $60km/h\times 2h=120km$

Now we need to calculate the time with which the calculated distance will be covered with a speed of 80 km/h.

Equation (i) can also be written as $Time=\dfrac{Dis\tan ce}{Speed}$

Distance we have is 120 km and the given speed is 80 km/h.

Therefore, $Time=\dfrac{120km}{80km/h}$

On simplifying we have,

$Time=\dfrac{3}{2}h=1.5h$

Hence, the answer is 1.5 hours.

Note: We must remember that the speed must be the same throughout the distance travelled. Hence, the speed must be uniform and if not the formula distance=speed x time cannot be used.

In the question it is not exactly mentioned that the speed is uniform throughout the journey and the distance travelled at first must be used to calculate the time when speed increases to 80 km/h. We must be intuitive and understand these things.

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