Question 508301
d = r*t is the basic distance equation, where d = distance, r= rate, and t = time
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We have two trips of equal distance here, but we don't know the distance.
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x = time going to the island
and
11 = rate of speed going to the island
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We know the two trips took 10.5 hr in total
so
10.5-x = time coming back from the island
and
10 = rate of speed coming back
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d=d
so
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11x = 10*(10.5-x)
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11x = 105 -10x
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21x = 105
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x = 105/21
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x = 5 hr
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d = 11 * 5 = 55
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Check using the return trip.
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10.5 -5 = 5.5
10 * 5.5 = 55
Correct.
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Answer:  The island is 55 knots from the harbor.
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Note that the unit of measurement 'knots' can mean nautical miles and nautical miles per hr.
In this case, both meanings are knot are used.
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Done.