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# A motorboat, which is set to travel at k kilometers per hour in still

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Re: A motorboat, which is set to travel at k kilometers per hour in still [#permalink]
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x = speed of the current

k + x = speed of the boat going downstream

k - x = speed of the boat going upstream (against the current)

(1) It takes the same amount of time for the boat to travel 4 kilometers directly downstream as it takes for it to travel 3 kilometers directly upstream.

3 (k + x) = 4 (k - x)
7x = k

We have two variables but only one equation. Not sufficient.

(2) The current flows directly downstream at a constant rate of 2.5 kilometers per hour.

x = 2.5. Not sufficient to determine k.

(1&2) 7(2.5) = k. Sufficient.
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Re: A motorboat, which is set to travel at k kilometers per hour in still [#permalink]
Hi. Why did we assume the boat's speed would be greater than that of the stream?
If current speed is greater than boat speed,
4 (current speed - boat speed ) = 3 (current speed + boat speed)
So, ratio of current speed to boat speed could be the reciprocal, 2.5/7.

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Re: A motorboat, which is set to travel at k kilometers per hour in still [#permalink]
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Re: A motorboat, which is set to travel at k kilometers per hour in still [#permalink]
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