A boat running upstream takes 640 min to cover a certain distance, while it takes 280 min to cover the same distance running down stream. What is the ratio between the speed of the boat and speed of water current respectively?	Answers
A) 23 : 7  B) 22 : 9  C) 23 : 8  D) 23 : 9  E) 23 : 11

Rationale: let the speed of the boat = x and speed of the current = y relative speed in upstream = x - y and in downstream = x + y let the distance ' d ' is to be covered in downstream and upstream , then time taken in upstream = d / ( x - y ) = 640 or d = 640 ( x - y ) - - - - ( i ) and time taken in downstream = d / ( x + y ) = 280 or d = 280 ( x + y ) - - - - ( ii ) comparing ( i ) & ( ii ) , 640 ( x - y ) = 280 ( x + y ) = > 16 ( x - y ) = 7 ( x + y ) = > 16 x - 16 y = 7 x - 7 y = > 9 x = 23 y = > x / y = 23 / 9 answer : d
Answer: D