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1). 3 hours and 30 minutes2). 4 hours3). 4 hours and 15 minutes4). 4 hours and 20 minutes |
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Answer» Let the speed of the stream be V km/hr. Upstream speed = (15 – V) km/hr Downstream speed = (15 + V) km/hr 6 hours and 30 minutes is 6.5 hours. To cover 78 upstream, 6.5 hours are required. We know, Distance = Speed × Time ⇒ 78 = (15 – V) × 6.5 ⇒ V = 15 – (78/6.5) = 15 – 12 = 3 ⇒ Downstream speed = (15 + V) km/hr = 18 km/hr ∴ Time taken to cover 78 km downstream = 78/18 = 4.333 Time inhours = 4 hours + (1/3) hours = 4 hours and 20 minutes |
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