# Moderate Algebra Solved QuestionAptitude Discussion

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.

 Q. Common Information Question: 2/2 The total cost for the journey at this most economical speed is:
 ✖ A. Rs 6,000 ✔ B. Rs 8,000 ✖ C. Rs 10,000 ✖ D. Rs 11,000

Solution:
Option(B) is correct

Cost of fuel is proportional to square of the speed:

$E=KS^2$

$\Rightarrow E=64 \text{ and } S=16$

$K=\dfrac{1}{4}$

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

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

$=\text{Rs }8,000$

## (1) Comment(s)

Bhumit
()

It should be 4000 + 400 = 4400 !!