A river boat leaves Silver Town and travels upstream to Gold Town at a

Question

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

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

nitamathew · Accepted Answer

Pick a number which is LCM of 9 and 6 = 18. 
Upstream time = 18/6 = 3 hrs
Downstream time = 18/9 = 2 hrs
Total time = 5 hrs
Total distance = 36
Average Speed = 36/5 = 7.2 km/hr

gracie · Answer

let total distance=2d
total time=d/6+d/9=5d/18
2d/(5d/18)=36/5=7.2 km/hr

wolves5 · Answer

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

ScottTargetTestPrep · Answer

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

KanishkM · Answer

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

= 2 * 6 * 9 / 15
= 7.2

C

AkshdeepS · Answer

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)

bumpbot · Answer

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A river boat leaves Silver Town and travels upstream to Gold Town at a

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