Shortcut Trick
Downstream speed (D) : Upstream speed (U) = Distance Ratio (in same time) = 10 : 4 = 5 : 2.
Let D = 5k and U = 2k. Total time = (30 ÷ 5k) + (30 ÷ 2k) = 14 ⇒ 6/k + 15/k = 14 ⇒ 21/k = 14 ⇒ k = 3/2.
Speed of stream = (D − U) ÷ 2 = (5k − 2k) ÷ 2 = 3k/2.
Substitute k = 3/2 ⇒ Speed of stream = (3 × 3/2) ÷ 2 = 9/4 km/h.
∴ The correct answer is 9/4 km/h.
Alternate Method
Given:
Total distance for a one-way trip = 30 km.
Total time for the round trip = 14 hours.
Distance covered downstream in a given time = 10 km.
Distance covered upstream in the same time = 4 km.
Formula Used:
Time = Distance ÷ Speed
Speed of stream = (Downstream Speed − Upstream Speed) ÷ 2
Calculations:
Let the speed of the boat in still water be u km/h and the speed of the stream be v km/h.
⇒ Downstream speed (D) = u + v
⇒ Upstream speed (U) = u − v
Since the time taken is the same for 10 km downstream and 4 km upstream:
⇒ 10 ÷ (u + v) = 4 ÷ (u − v)
⇒ (u + v) ÷ (u − v) = 10/4 = 5/2
Let Downstream speed D = 5k and Upstream speed U = 2k.
Total time taken for the round trip is 14 hours:
⇒ 30 ÷ D + 30 ÷ U = 14
⇒ 30 ÷ 5k + 30 ÷ 2k = 14
⇒ 6/k + 15/k = 14
⇒ 21/k = 14
⇒ k = 21 ÷ 14 = 3/2
Now, calculate the absolute speeds D and U:
⇒ Downstream Speed (D) = 5 × 3/2 = 15/2 = 7.5 km/h
⇒ Upstream Speed (U) = 2 × 3/2 = 3 km/h
Speed of the stream (v) = (D − U) ÷ 2
⇒ v = (7.5 − 3) ÷ 2 = 4.5 ÷ 2 = 9/4 km/h
∴ The correct answer is 9/4 km/h.
Additional Information
Downstream and Upstream Speeds
When a boat moves with the stream, its effective speed increases (D = u + v). When it moves against the stream, its effective speed decreases (U = u − v).
Boat in Still Water
The speed of a boat in still water (u) can be calculated directly if downstream and upstream speeds are known: u = (D + U) ÷ 2.
Speed of the Stream
The speed of the stream or current (v) is half the difference between the downstream and upstream speeds: v = (D − U) ÷ 2.
Constant Time Principle
If the time taken to travel two different distances is the same, the ratio of the distances is directly proportional to the ratio of their respective speeds (D1 ÷ D2 = S1 ÷ S2).