A ship takes 15 hours for travelling D km downstream and (D + 15) km upstream. If the ship takes 13 hours for covering (D – 15) km downstream and (D + 10) km upstream and the speed of stream is 5 km/hr then, find time taken by the ship to cover (D + 60) km in downstream?
- AOption (a)
5 hours
- BOption (b)
6 hours
- COption (c)
7 hours
- DOption (d)
8 hours
- EOption (e)
None of these
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Correct answer
7 hours
Why this answer is correct
Shortcut Trick
Let Downstream speed = u and Upstream speed = v.
Case 1: D/u + (D+15)/v = 15 → (D/u + D/v) + 15/v = 15
Case 2: (D−15)/u + (D+10)/v = 13 → (D/u + D/v) − 15/u + 10/v = 13
Subtracting Case 2 from Case 1: 15/u + 5/v = 2. Since Speed of stream = 5, u − v = 10.
By trial/substitution: u = 15, v = 5 fits (15/15 + 5/5 = 2). Then D = 75 km.
Target Time = (75 + 60) ÷ 15 = 135 ÷ 15 = 9 hr. Check constraints or solve for D.
∴ The correct answer is 9 hr.
Alternate Method
Given: Speed of stream (y) = 5 km/hr. Let speed of ship in still water be x km/hr.
⇒ Downstream speed (u) = x + 5, Upstream speed (v) = x − 5.
Equation 1: D/(x+5) + (D+15)/(x−5) = 15
Equation 2: (D−15)/(x+5) + (D+10)/(x−5) = 13
⇒ Subtracting (2) from (1): [D − (D−15)]/(x+5) + [(D+15) − (D+10)]/(x−5) = 15 − 13
⇒ 15/(x+5) + 5/(x−5) = 2
⇒ 15(x−5) + 5(x+5) = 2(x2 − 25)
⇒ 15x − 75 + 5x + 25 = 2x2 − 50 ⇒ 20x − 50 = 2x2 − 50
⇒ 2x2 − 20x = 0 ⇒ 2x(x − 10) = 0 ⇒ x = 10 km/hr (since speed cannot be 0)
⇒ u = 10 + 5 = 15 km/hr and v = 10 − 5 = 5 km/hr.
⇒ Substitute x in (1): D/15 + (D+15)/5 = 15 ⇒ D + 3D + 45 = 225 ⇒ 4D = 180 ⇒ D = 45 km.
⇒ Target distance = D + 60 = 45 + 60 = 105 km.
⇒ Time = 105 ÷ 15 = 7 hr.
∴ The correct answer is 7 hr.
Additional Information
Downstream & Upstream Speeds
If boat speed is x and stream speed is y, then Downstream = x + y and Upstream = x − y.
Still Water Speed Calculation
Speed in still water (x) = (Downstream + Upstream) ÷ 2.
Stream Speed Calculation
Speed of stream (y) = (Downstream − Upstream) ÷ 2.
