Boat travelling upstream : GMAT Problem Solving (PS)
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Boat travelling upstream

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Boat travelling upstream [#permalink]

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New post 16 Mar 2011, 00:11
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  35% (medium)

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90% (04:08) correct 10% (04:52) wrong based on 10 sessions

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Pls refer to attached picture for the question

Apparently you need to approximate well enough to get the correct answer. I was wondering if anyone could provide a better way of doing this since I could get the equation but couldn't produce a precise answer under test stress. :/
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Re: Boat travelling upstream [#permalink]

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New post 16 Mar 2011, 00:23
It just a quadratic equation.

The travel upstream longs for 90/(v-3) hours, the travel upstream - 90/(v+3). Since it is half an hour shorter, ther:
90/(v-3)=90/(v+3)+0.5
90(v+3)=90(v-3)+0.5(v^2-9)
0.5v^2=270*2+9*0.5
v^2=270*4+9
v^2=800+280+9=1089
v=33

To travel downstream it takes 90/(33+3)=90/36=10/4=2.5 hours
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Re: Boat travelling upstream [#permalink]

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New post 16 Mar 2011, 00:27
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drifting wrote:
Pls refer to attached picture for the question

Apparently you need to approximate well enough to get the correct answer. I was wondering if anyone could provide a better way of doing this since I could get the equation but couldn't produce a precise answer under test stress. :/


The equation is 90/(v-3) - 90/(v+3) = 0.5

If we solve for v, we can get the answer

A simple rearrangement would give (v+3)*(v-3) = 1080=36*30 Hence v+3 = 36 and v-3 = 30 or v=33, so times for downstream = 90/36=2.5
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Re: Boat travelling upstream [#permalink]

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New post 16 Mar 2011, 00:50
beyondgmatscore wrote:
drifting wrote:
Pls refer to attached picture for the question

Apparently you need to approximate well enough to get the correct answer. I was wondering if anyone could provide a better way of doing this since I could get the equation but couldn't produce a precise answer under test stress. :/


The equation is 90/(v-3) - 90/(v+3) = 0.5

If we solve for v, we can get the answer

A simple rearrangement would give (v+3)*(v-3) = 1080=36*30 Hence v+3 = 36 and v-3 = 30 or v=33, so times for downstream = 90/36=2.5


I see.. I made a mistake when trying to for V and ended up getting V=34 instead. Thanks!
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Re: Boat travelling upstream [#permalink]

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New post 17 Mar 2011, 00:09
90/(v-3) = 90/(v+3) + 1/2


=> 90(v+3 - v +3)/(v-3)(v+3) = 1/2

180 * 6 = v^2 - 9

=> v^2 = 9(20*6 + 1)

=> V = 3*11 = 33

So Downstream time = 90/36 = 30/12 = 2.5

The answer is A.
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Re: Boat travelling upstream   [#permalink] 17 Mar 2011, 00:09
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