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A motor boat, whose speed is 15 km/hr in still water goes 30 km downstream and comes back in a total of 4 hrs 30 mins. The speed of the stream (in km/hr) is:-1. 5 km/hr.2. 10 km/hr.3. 15 km/hr.4. 20 km/hr.5. 25 km/hr. |
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Answer» Correct Answer - Option 1 : 5 km/hr. Given: Speed in still water = 15 km/hr Total distance = 30 km Total time = 4 hrs 30 mins = 412 hr = 9/2 hr. Formula used: Distance = Speed × Time Calculation: Let the speed of the of the stream be X km/hr Speed upstream = (15 – X) km/hr Speed downstream = (15 + X) km/hr According to question: ⇒ [30/(15 + x) + 30/(15 – X)] = 9/2 ⇒ (450 – 30X + 450 + 30X) / (15 + X) × (15 – X) = 9/2 ⇒ 900 = [(152 – X2) × 9/2] ⇒ (900 × 2/9) = 225 – X2 ⇒ X2 = 225 – 200 ⇒ X2 = 25 ⇒ x = 5 ∴ Speed of the stream is 5 km/hr. If the speed of boat or swimmer is X km/hr and the speed of the stream is y km/hr then, Speed of boat upstream = (X – Y) km/hr. Speed of boat downstream = (X + Y) km/hr. |
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