The speed at which a man can row a boat in still water is 15 kmph. If he rows downstream, where the speed of current is 3 kmph, what time will he take to cover 100 metres?  Choose the correct choice from the following answers
A) 16 seconds  B) 20 seconds  C) 14 seconds  D) 12 seconds  E) 15 seconds

Rationale: "speed of the boat downstream = 15 + 3 = 18 kmph = 18 * 5 / 18 = 5 m / s hence time taken to cover 100 m = 100 / 5 = 20 seconds . answer : b"
Answer: B