Subject: mathqa
Question: 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 9 coaches are attached speed decreases to 78 kmph . what will be the speed of train when 25 coaches are attached .
Option: 
A: 85
B: 60
C: 70
D: 80
Answer: C