Given upstream time = 2 × downstream time Concept When the distance travelled is the same in upstream and downstream, speed is inversely proportional to the time taken. Calculation let speed of boat in still water be B Let speed of stream be S Upstream time = distance/upstream speed ⇒ upstream time = distance/(B – S) ----(1) Downstream time = distance/ downstream speed ⇒ downstream time = distance/(B + S) ----(2) Applying the given condition to 1 and 2, ⇒ distance/(B – S) = 2 × distance/(B + S) ⇒ B + S = 2 × (B – S) ⇒ 3S = B ⇒ S = B/3 ----(3) As per question, when we double the speed of boat in still water, we get B’ = 2B Then, upstream speed = B’ + S ⇒ upstream speed = 2B + S From (3), we can write upstream speed = 2B – B/3 Similarly, downstream speed = 2B + B/3 Then, ratio of upstream speed to downstream speed = (2B – B/3)/(2B + B/3) Since we know that when distance is same, time is inversely proportional to speed we can write Upstream time ∶ downstream time = (2B + B/3) ∶ (2B – B/3) ⇒ upstream time ∶ downstream time = 7 ∶ 5 ∴ Ratio of upstream to downstream time is 7 ∶ 5 .