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A boat goes 8 km upstream and 8 km downstream in 3 hours. If the speed of current is 2kmph, find the speed of boat in still water in kmph.
1. 4
2. 3
3. 6
4. 8

1 Answer

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Correct Answer - Option 3 : 6

Given:

A boat goes 8 km upstream and 8 km downstream in 3 hours

The speed of the current is 2kmph

Formula Used:

Time = Distance/Speed

Speed of upstream = (x – y)

Speed of downstream = (x + y)

Where, x = Speed of boat in still water

y = Speed of current

Calculation:

According to the question

[8/(x - 2)] + [8/(x + 2)] = 3

⇒ 2x/(x2 – 4) = 3/8

⇒ 16x = 3x2 – 12

⇒ 3x2 - 16x - 12 = 0

⇒ 3x2 - 18x + 2x - 12 = 0

⇒ 3x(x - 6) + 2( x - 6) = 0

⇒ (3x + 2) (x - 6) = 0

⇒ x = -2/3 (not possible) or x = 6 

∴ x = 6 kmph = speed of boat in still water.

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