An engine moves at the speed of 90 kmph without any coaches attached to it.Speed of the train reduces at the rate that varies directly as the square root of the number of coaches attached.When 4 coaches are attached speed decreases to 78 kmph.What will be the speed of train when 16 coaches are attached.  Choose the correct choice from the following.
A) 66	B) 72	C) 80	D) 68	E) 73

Rationale: "1 . no . of coaches = 4 sqr root = 2 speed decreases by 12 12 = k * 2 k = 6 no . of coaches = 16 swr root = 4 decrease = 4 * 6 = 24 new speed = 90 - 24 = 66 a"
Answer: A