Shortcut Trick
Divide first case by 20: 3/D + 2/U = 0.625
Divide second case by 21: 4/D + 3/U = 0.90
Eliminate 1/D: 3 × (4/D + 3/U) − 4 × (3/D + 2/U) = 3(0.9) − 4(0.625)
1/U = 2.7 − 2.5 = 0.2 ⇒ Upstream speed (U) = 5 km/h
3/D + 2(0.2) = 0.625 ⇒ 3/D = 0.225 ⇒ Downstream speed (D) = 13.33 km/h
Boat speed = (13.33 + 5) ÷ 2 = 9.165 ≈ 9 km/h
∴ The correct option is C.
Alternate Method
Given:
Case 1: Distance downstream = 60 km, Distance upstream = 40 km, Time = 12.5 hours
Case 2: Distance downstream = 84 km, Distance upstream = 63 km, Time = 18.9 hours (as 54/60 = 0.9)
Formula:
Time = Distance ÷ Speed | Speed of boat in still water (x) = (D + U) ÷ 2
Boat Speed (x) = (D + U) ÷ 2
Downstream: x + y (Speed D)
Upstream: x − y (Speed U)
⇒ Let 1/D = a and 1/U = b
⇒ 60a + 40b = 12.5 --- (i)
⇒ 84a + 63b = 18.9 --- (ii)
⇒ Multiply (i) by 1.4: 84a + 56b = 17.5
⇒ Subtracting from (ii): (84a + 63b) − (84a + 56b) = 18.9 − 17.5
⇒ 7b = 1.4 ⇒ b = 0.2
⇒ U = 1/b = 1/0.2 = 5 km/h
⇒ 60a + 40(0.2) = 12.5 ⇒ 60a + 8 = 12.5
⇒ 60a = 4.5 ⇒ a = 0.075
⇒ D = 1/a = 1/0.075 = 13.33 km/h
⇒ Speed of boat (x) = (13.33 + 5) ÷ 2 = 18.33 ÷ 2 = 9.165 km/h
⇒ Nearest integer = 9 km/h
∴ The correct option is C.
Additional Information
Downstream and Upstream Speeds
If speed of boat is x and stream is y, then Downstream (D) = x + y and Upstream (U) = x − y.
Calculating Still Water and Stream Speed
Boat speed in still water x = (D + U) ÷ 2; Speed of stream y = (D − U) ÷ 2.
Time Conversion
To convert minutes to hours, divide by 60. Example: 54 minutes = 54/60 = 0.9 hours.