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The speed of the current is 5 km/hour. A motorboat goes 10 km upstream and back again to the starting point in 50 minutes. The speed (in km/hour) of the motorboat in still water is
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- 20
- 26
- 25
- 28
- 20
Correct Option: C
Let the speed of motorboat in still water be p kmph.
| ∴ | + | = | |||
| p - 5 | p + 5 | 60 |
| ⇒ 10 |  |  | = | ||
| (p + 5)(p - 5) | 6 |
⇒ 20p × 6 = (p² – 25) × 5
⇒ p² – 24p – 25 = 0
⇒ p² – 25p + p – 25 = 0
⇒ p(p – 25) + 1 (p – 25) = 0
⇒ (p – 25) (p + 1) = 0
⇒ p = 25 because p ≠ –1
Hence , Speed of motorboat in still water = 25 kmph
