Two trains, one from Howrah to Patna and the other from Patna to Howrah, start simultaneously. After they meet, the trains reach their destinations after 4 hours and 36 hours respectively. The ratio of their speeds is? Select.
A) 4 : 9 B) 4 : 3 C) 4 : 5 D) 3 : 1 E) 4 : 2

Rationale: "let us name the trains a and b . then , ( a ' s speed ) : ( b ' s speed ) = √ b : √ a = √ 36 : √ 4 = 6 : 2 = 3 : 1 answer : d"
Answer: D