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Option 4 : 15 kmph
Given:
A boat goes 14 km upstream and 15 km downstream
Speed of stream = 4 kmph
Total time taken by boat = 3 hours
Formula Used:
Speed of boat in downstream = speed of (boat + stream)
Speed of boat in upstream = speed of (boat - stream)
Calculation:
Let the speed of boat in still water be A kmph.
Speed downstream = A + 4
Speed upstream = A – 4
Total time = 3 hours
Given,
⇒ 15/(A + 4) + 14/(A – 4) = 3
⇒ 15A – 60 + 14A + 56 = 3A2 – 48
⇒ 3A2 – 29A – 44 = 0
Solving,
⇒ A = 11
Speed of boat in still water = 11 kmph
∴ Speed downstream = 11 + 4 = 15 kmph
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