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Common Information

The cost of fuel for running the engine of an army tank is proportional to the square of the speed and Rs. 64/hour for a speed of 16 kmph. Other costs amount to Rs. 400/hour. The tank has to make a journey of 400 km at a constant speed.


Common Information Question: 2/2

The total cost for the journey at this most economical speed is:


Rs 6,000


Rs 8,000


Rs 10,000


Rs 11,000

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Option(B) is correct

Cost of fuel is proportional to square of the speed:


\(\Rightarrow E=64 \text{ and } S=16 \)


Total cost 

\(=\dfrac{1}{4}\times S^2t+400t\)

Most economical speed, checking options we get most economical speed at 40km/hr.

Total cost at 40km/hr

\(=\dfrac{1}{4}\times 40^2+400\left(\dfrac{400}{40}\right)\)

\(=\text{Rs }8,000\)

(2) Comment(s)

Siddhi Verma

It should be 400+4000=4400


It should be 4000 + 400 = 4400 !!