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# A Banana Boat travels from Sunniville to Gloomiville

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Senior Manager
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A Banana Boat travels from Sunniville to Gloomiville [#permalink]  14 Jan 2004, 19:54
A Banana Boat travels from Sunniville to Gloomiville upstream and returns back to Sunniville downstream. If the average speed of the total journey is 37.5 km/hr and the speed of the stream is 10 km/hr, what is the upstream speed of the Banana Boat?

a.40
b.50
c.30
d.27.5
e.35
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30 Kmph. its the same logic we used in the earlier similar problem "CRUISE 2"
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C Indeed. Here is the work though.
a-10 = upstream speed
a+10 = downstream speed
2(a-10)(a+10) / (a-10)+(a+10) = 37.5
2(a^2-100) / 2a = 37.5
a^2 - 100 = 37.5a
a^2 - 37.5a - 100 = 0
(a-40)(a+2.5) = 0
a = 40
upstream speed = a - 10 = 40 - 10 = 30
Many cruises problems lately
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I was wondering where the formula for average speed came from?

I've never learned it before?
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Hi Calnhob, I learnt it in India but I am in Australia now. Anyways it is 2ab/a+b for average speed. Just memorise it like how I did. Do u find it useful. I think it saves lot of time.
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U can even derive the formula by taking two variables.
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Excuse me sunnyboy, are you this hot girl in the picture? I really feel inspired while study your questions!
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Senior Manager
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dflamen, if thinking that I am the hot chick in the picture will help you ace the GMAT...please do go ahead and assume so...
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Pls include reasoning along with all answer posts.
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Hey rakesh1239, yeah the formula is great. I dont remember seeing it in school but I was able to finally derive it after pluging in variables.
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