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A boat goes 12 km downstream and comes back to the starting point in 3 hours. If the speed of the current is 3 km/hr, then the speed (in km/hr) of the boat in still water is
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- 12
- 9
- 8
- 6
- 12
Correct Option: B
Let the speed of boat in still water be y kmph, then
| + | = 3 | ||
| y + 3 | y - 3 |
| ⇒ 12 |  |  | = 3 | |
| (y + 3)(y -3) |
⇒ 4 × 2y = y² – 9
⇒ y² – 8y – 9 = 0
⇒ y² – 9y + y – 9 = 0
⇒ y (y – 9) + 1 (y – 9) = 0
⇒ (y – 9) (y + 1) = 0
⇒ y = 9 because y ≠ –1
∴ Speed can't be negetive.
Hence, speed of boat in still water = 9 kmph
