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Re: A river boat leaves Silver Town and travels upstream to Gold Town at a [#permalink]
Distances are going to be the same since it is a round-trip. Find an arbitrary value for the distance that can be used by both rates…aka find the multiple. 18 km works.

Then find the hours using D=RT.
18km = (9km/hr)(time) --> 2 hours
18km = (6km/hr)(time) --> 3 hours

Find the total distance and time to get the average speed.
(18km + 18km) / (2 hours + 3 hours) = 36/5 = 7.2 km/hrs

Answer: C
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Re: A river boat leaves Silver Town and travels upstream to Gold Town at a [#permalink]
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Bunuel wrote:
A river boat leaves Silver Town and travels upstream to Gold Town at an average speed of 6 kilometers per hour. It returns by the same route at an average speed of 9 kilometers per hour. What is the average speed for the round-trip in kilometers per hour?

A) 7.0
B) 7.1
C) 7.2
D) 7.5
E) 8.0


We can let the distance = 18, so that the time upstream is 18/6 = 3 hours and the time downstream is 18/9 = 2 hours. Now we can determine average rate.

average rate = total distance/total time

average rate = 36/(3 + 2) = 36/5 = 7.2

Answer: C
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Re: A river boat leaves Silver Town and travels upstream to Gold Town at a [#permalink]
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Bunuel wrote:
A river boat leaves Silver Town and travels upstream to Gold Town at an average speed of 6 kilometers per hour. It returns by the same route at an average speed of 9 kilometers per hour. What is the average speed for the round-trip in kilometers per hour?

A) 7.0
B) 7.1
C) 7.2
D) 7.5
E) 8.0


Average Speed = 2 a * b/ (a+b)

= 2 * 6 * 9 / 15
= 7.2

C
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A river boat leaves Silver Town and travels upstream to Gold Town at a [#permalink]
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Bunuel wrote:
A river boat leaves Silver Town and travels upstream to Gold Town at an average speed of 6 kilometers per hour. It returns by the same route at an average speed of 9 kilometers per hour. What is the average speed for the round-trip in kilometers per hour?

A) 7.0
B) 7.1
C) 7.2
D) 7.5
E) 8.0


Note : When distances of both the legs are equal, the average speed equals

Avg. speed = \(\frac{2*a*b}{(a + b)}\)

Where "a" and "b" are individual speeds given.

Therefore, \(\frac{2*6*9}{6+9}\)

Answer : 7.2 kmph (C)
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Re: A river boat leaves Silver Town and travels upstream to Gold Town at a [#permalink]
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Re: A river boat leaves Silver Town and travels upstream to Gold Town at a [#permalink]
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