The cost of fuming the engine of an army tank is proportional to the square of the speed and Rs 64 per hour for a speed of 16km/hr. Other costs amount to Rs 400 per hour. The tank has to make a journey of 400km at a constant speed.The most economical speed for this journey is :	Choose the correct choice from the following answers.
A) 20 km/hr  B) 30 km/hr  C) 35 km/hr  D) 40 km/hr  E) None of these
Explanation :   Cost = k2×speed2[ Where k is constant], And   =&gt;64 = k×16×16   =&gt;k=1/4.   =&gt;cost=speed2 /4.   Total cost=[(speed2/4)+400]×400/speed   using options, putting different values of speed, we find speed = 40km/hr to be most economical.   Answer : 40 km/hr
<result>D</result>
The cost of fuming the engine of an army tank is proportional to the square of the speed and Rs 64 per hour for a speed of 16km/hr. Other costs amount to Rs 400 per hour. The tank has to make a journey of 400km at a constant speed.The most economical speed for this journey is :