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A boat travels downstream and upstream for 24 km each. If the speed of boat in still water is 7 km/hr and it takes 14 hours to complete the whole journey, then what is the upstream speed?1. 2 km/hr2. 6 km/hr3. 14 km/hr4. 10 km/hr |
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Answer» Correct Answer - Option 1 : 2 km/hr Given: Downstream distance = 24 km Upstream distance = 24 km Formula Used: Speed = Distance/Time D = B + S U = B - S where, D → Downstream speed, U → Upstream speed, B → Speed of the boat in still water, S → Speed of the stream. Calculations: 24/D + 24/U = 14 ⇒ 24/(B + S) + 24/(B - S) = 14 ⇒ 24/(7 + S) + 24/(7 - S) = 14 ⇒ [24 × (7 - S) + 24 × (7 + S)]/(7 + S) × (7 - S) = 14 ⇒ 24 × 7 × 2/(49 - S2) = 14 ⇒ 24 = 49 - S2 ⇒ S2 = 49 - 24 = 25 ⇒ S = 5 km/hr U = (B - S) ⇒ U = (7 - 5) ⇒ U = 2 km/hr ∴ The upstream speed is 2 km/hr. |
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