Shortcut Trick
Boat speed in still water = 30 ÷ 6 = 5 km/hr.
Condition: 72 ÷ (5 + v) + 18 ÷ (5 − v) = 26 hr, where v is stream speed.
Test options: If v = 4, then 72 ÷ (5 + 4) + 18 ÷ (5 − 4) = 72 ÷ 9 + 18 ÷ 1 = 8 + 18 = 26 hr.
The condition is satisfied for v = 4.
∴ The correct answer is 4 km/hr.
Alternate Method
Given:
Distance in still water = 30 km, Time = 6 hr.
Downstream distance = 72 km, Upstream distance = 18 km, Total time = 26 hr.
Formula Used:
Speed = Distance ÷ Time
Downstream Speed (d) = u + v
Upstream Speed (s) = u − v
Calculations:
⇒ Speed of boat in still water (u) = 30 ÷ 6 = 5 km/hr
⇒ Let stream speed be v km/hr
⇒ 72 ÷ (5 + v) + 18 ÷ (5 − v) = 26
⇒ [72(5 − v) + 18(5 + v)] ÷ [(5 + v)(5 − v)] = 26
⇒ [360 − 72v + 90 + 18v] ÷ (25 − v2) = 26
⇒ 450 − 54v = 26(25 − v2)
⇒ 450 − 54v = 650 − 26v2
⇒ 26v2 − 54v − 200 = 0
⇒ 13v2 − 27v − 100 = 0
⇒ 13v2 − 52v + 25v − 100 = 0
⇒ 13v(v − 4) + 25(v − 4) = 0
⇒ (13v + 25)(v − 4) = 0
⇒ v = 4 (as speed cannot be negative)
∴ The correct answer is 4 km/hr.
Additional Information
Downstream and Upstream Speeds
Downstream speed (D) = u + v and Upstream speed (U) = u − v, where u is boat speed in still water and v is stream speed.
Speed of Boat and Stream
Boat speed in still water u = (D + U) ÷ 2 and Stream speed v = (D − U) ÷ 2.
Time, Speed and Distance Relation
Time = Distance ÷ Speed. When distance is constant, speed is inversely proportional to time.