Two trains started at the same time, one from A to B and other from B to A. if they arrived at B and A respectively in 4 hours and 9 hours after they passed each other, the ratio of the speeds of the two trains was:
A. 2 : 1
B. 3 : 2
C. 4 : 3
D. 5 : 4
Answer: Option B

v/V=V/v(9/4)
by what rules?