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# ds rate (m08q34)

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ds rate (m08q34) [#permalink]

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28 Dec 2007, 11:06
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A plane takes off from a hill at 750 meters above sea level and lands some time later in a town located at 50 meters below sea level. During the first part of its flight, the plane gained height at a rate of 50 meters per minute but then it started to descend at a rate of 20 meters per minute. The duration of the first part of the flight was what percent of the total flight time?

1. The duration of the descent is known.
2. The total flight time is known.

[Reveal] Spoiler: OA
D

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Re: ds rate [#permalink]

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28 Dec 2007, 11:26
bmwhype2 wrote:
a plane takes off from a hill at 750 meters above sea level and lands some time later in a town located at 50 meters below sea level. During the first part of its flight the plane gained height at 50 meters per minute but then it descended at a rate of 20 meters per minute. The duration of the first part of the flight was what percent of the total flight time?

1. during of descent is known
2. total flgiht time is known

(1) Duration of decent will let us know the highest point the plane was at.
So by sybstracting 750 from highest point we know how much it climed in acent. So we can figure out time for ascent. Thus we can calcuate ratio

Suff

2 Insuff
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28 Dec 2007, 12:06
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Expert's post
D

general equation: 750-50=20*td-50ta ==> 2*td-5*ta=70
where ta - time of ascent, td - time of descent

1. we know td. we can find ta from general equation. SUFF.
2. we know ta+td. we can solve system of 2 equations. SUFF.
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15 Oct 2009, 19:51
walker wrote:
D

general equation: 750-50=20*td-50ta ==> 2*td-5*ta=70
where ta - time of ascent, td - time of descent

1. we know td. we can find ta from general equation. SUFF.
2. we know ta+td. we can solve system of 2 equations. SUFF.

Thx... I was wondering why it was 750+50 in explanation..
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Re: ds rate (m08q34) [#permalink]

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15 Oct 2009, 22:23
3
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bmwhype2 wrote:
A plane takes off from a hill at 750 meters above sea level and lands some time later in a town located at 50 meters below sea level. During the first part of its flight, the plane gained height at a rate of 50 meters per minute but then it started to descend at a rate of 20 meters per minute. The duration of the first part of the flight was what percent of the total flight time?

1. The duration of the descent is known.
2. The total flight time is known.

mandb wrote:
walker wrote:
D

general equation: 750-50=20*td-50ta ==> 2*td-5*ta=70
where ta - time of ascent, td - time of descent

1. we know td. we can find ta from general equation. SUFF.
2. we know ta+td. we can solve system of 2 equations. SUFF.

Thx... I was wondering why it was 750+50 in explanation..

It is.

Present height + (+ accent) - (- level below the sea) = 20d
750 + 50a - (-50) = 20d
75 + 5a + 5 = 2d
80 + 5a = 2d

St. 1) The duration of the descent (i.e. d) is known.
Say d = 50, a = 4. SUFF...

St. 2) The total flight (i.e. a+d) time is known.
For simplicity, say a+d = 54
a = 54 - d

Then, 80 + 5(54-d) = 2d.
d = 50. SUFF.

D
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Re: ds rate (m08q34) [#permalink]

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16 Oct 2009, 15:47
GMAT TIGER wrote:
bmwhype2 wrote:
A plane takes off from a hill at 750 meters above sea level and lands some time later in a town located at 50 meters below sea level. During the first part of its flight, the plane gained height at a rate of 50 meters per minute but then it started to descend at a rate of 20 meters per minute. The duration of the first part of the flight was what percent of the total flight time?

1. The duration of the descent is known.
2. The total flight time is known.

mandb wrote:
walker wrote:
D

general equation: 750-50=20*td-50ta ==> 2*td-5*ta=70
where ta - time of ascent, td - time of descent

1. we know td. we can find ta from general equation. SUFF.
2. we know ta+td. we can solve system of 2 equations. SUFF.

Thx... I was wondering why it was 750+50 in explanation..

It is.

Present height + (+ accent) - (- level below the sea) = 20d
750 + 50a - (-50) = 20d
75 + 5a + 5 = 2d
80 + 5a = 2d

St. 1) The duration of the descent (i.e. d) is known.
Say d = 50, a = 4. SUFF...

St. 2) The total flight (i.e. a+d) time is known.
For simplicity, say a+d = 54
a = 54 - d

Then, 80 + 5(54-d) = 2d.
d = 50. SUFF.

D

Oh I see one is above sea level and other one is below
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Re: ds rate (m08q34) [#permalink]

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07 Jun 2010, 09:53
3
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bmwhype2 wrote:
A plane takes off from a hill at 750 meters above sea level and lands some time later in a town located at 50 meters below sea level. During the first part of its flight, the plane gained height at a rate of 50 meters per minute but then it started to descend at a rate of 20 meters per minute. The duration of the first part of the flight was what percent of the total flight time?

1. The duration of the descent is known.
2. The total flight time is known.

let duration of ascend = u mins [up]
let duration of descend = d mins [down]

from the information we can write the below equation
50u+20d=800

(1) if d is known, we can calculate u's value
(2) if u+d is known, then we'll have two equations and we can solve them to get u's value

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Re: ds rate (m08q34) [#permalink]

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29 Oct 2010, 04:35
dimitri92 wrote:
bmwhype2 wrote:
A plane takes off from a hill at 750 meters above sea level and lands some time later in a town located at 50 meters below sea level. During the first part of its flight, the plane gained height at a rate of 50 meters per minute but then it started to descend at a rate of 20 meters per minute. The duration of the first part of the flight was what percent of the total flight time?

1. The duration of the descent is known.
2. The total flight time is known.

let duration of ascend = u mins [up]
let duration of descend = d mins [down]

from the information we can write the below equation
50u+20d=800

(1) if d is known, we can calculate u's value
(2) if u+d is known, then we'll have two equations and we can solve them to get u's value

I donot understand why vertical height is being considered here. A plane will never travel vertically down. The path followed is the hypotenuse of the triangle formed by the ground distance covered and the vertical height. The descent distance will definitely be greater than the vertical distance. Shouldnt we try to form a triangle to solve this? Plz help me out here.
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Re: ds rate (m08q34) [#permalink]

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29 Oct 2010, 05:11
5
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bmwhype2 wrote:
A plane takes off from a hill at 750 meters above sea level and lands some time later in a town located at 50 meters below sea level. During the first part of its flight, the plane gained height at a rate of 50 meters per minute but then it started to descend at a rate of 20 meters per minute. The duration of the first part of the flight was what percent of the total flight time?

1. The duration of the descent is known.
2. The total flight time is known.

[Reveal] Spoiler: OA
D

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Say the plane ascended by X meter @ 50mpm and descended (X+750+50) = (X+800) meter @ 20 mpm.
Time to go up, Tu = X/50 min ------------------------ eq1
Time to go down, Td = (X+800)/20 min. -------- eq2
Total time, Tt = Tu + Td = (7X+4000)/100 ----------- eq3

1) We have Td. So we can find out X from eq2 and plug in eq1 to get Tu. Sufficient.
2) We have Tt. So we can find out X from eq3 and plug in eq1 to get Tu. Sufficient.

D
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Re: ds rate (m08q34) [#permalink]

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29 Oct 2010, 05:18
BalakumaranP wrote:
dimitri92 wrote:
bmwhype2 wrote:
A plane takes off from a hill at 750 meters above sea level and lands some time later in a town located at 50 meters below sea level. During the first part of its flight, the plane gained height at a rate of 50 meters per minute but then it started to descend at a rate of 20 meters per minute. The duration of the first part of the flight was what percent of the total flight time?

1. The duration of the descent is known.
2. The total flight time is known.

let duration of ascend = u mins [up]
let duration of descend = d mins [down]

from the information we can write the below equation
50u+20d=800

(1) if d is known, we can calculate u's value
(2) if u+d is known, then we'll have two equations and we can solve them to get u's value

I donot understand why vertical height is being considered here. A plane will never travel vertically down. The path followed is the hypotenuse of the triangle formed by the ground distance covered and the vertical height. The descent distance will definitely be greater than the vertical distance. Shouldnt we try to form a triangle to solve this? Plz help me out here.

If the plane took T mins to reach from height X to ground, no matter what horizontal distance it covered, it would still have taken T mins to reach from hieght X to height 0.
Consider this on coordinate plane XY. Say the plane was initially at (1,10) and it took T min to reach (X,0). Now it matters not what the value of X is, since we know that displacement of body from Y=10 from Y=0 happened in T mins.

Makes sense??
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Re: ds rate (m08q34) [#permalink]

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29 Oct 2010, 05:35
mandb wrote:
Oh I see one is above sea level and other one is below

Thats the beauty of DS.
Even if you'd incorrectly assumed both were ABOVE sea level, you'd still have been able to answer the question.
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Re: ds rate (m08q34) [#permalink]

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29 Oct 2010, 06:08
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Yes.. now I get it.. The slope of the path taken upward and downward are perpendicular. Assuming this, we need not consider the horizontal distance covered.
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Re: ds rate (m08q34) [#permalink]

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29 Oct 2010, 06:46
bmwhype2 wrote:
A plane takes off from a hill at 750 meters above sea level and lands some time later in a town located at 50 meters below sea level. During the first part of its flight, the plane gained height at a rate of 50 meters per minute but then it started to descend at a rate of 20 meters per minute. The duration of the first part of the flight was what percent of the total flight time?

1. The duration of the descent is known.
2. The total flight time is known.

[Reveal] Spoiler: OA
D

Source: GMAT Club Tests - hardest GMAT questions

IMO this question is wrongly worded. By asking for the %, the answer should be sufficient if only we are able to give a number back....
therefore IMO rephrase the question or give some numbers to (1) and (2)

the question allows the answer to be different values...different values mean different answers therefore insufficient statements
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Re: ds rate (m08q34) [#permalink]

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29 Oct 2010, 07:06
zisis wrote:
bmwhype2 wrote:
A plane takes off from a hill at 750 meters above sea level and lands some time later in a town located at 50 meters below sea level. During the first part of its flight, the plane gained height at a rate of 50 meters per minute but then it started to descend at a rate of 20 meters per minute. The duration of the first part of the flight was what percent of the total flight time?

1. The duration of the descent is known.
2. The total flight time is known.

[Reveal] Spoiler: OA
D

Source: GMAT Club Tests - hardest GMAT questions

IMO this question is wrongly worded. By asking for the %, the answer should be sufficient if only we are able to give a number back....
therefore IMO rephrase the question or give some numbers to (1) and (2)

the question allows the answer to be different values...different values mean different answers therefore insufficient statements

I beg to differ.
DS questions do not ask us to find the answer, they just ask us whether or not it is possible to find the answer. Agreed, for different values of Descent Time and Total Time we'd get different values, but answer to DS question would always be same "Yes, we can find the answer" or "No, we can not find answer", whichever the case may be.
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Vaibhav

PS: Correct me if I am wrong.

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Re: ds rate (m08q34) [#permalink]

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29 Oct 2010, 07:49
vaibhavtripathi wrote:
zisis wrote:
bmwhype2 wrote:
A plane takes off from a hill at 750 meters above sea level and lands some time later in a town located at 50 meters below sea level. During the first part of its flight, the plane gained height at a rate of 50 meters per minute but then it started to descend at a rate of 20 meters per minute. The duration of the first part of the flight was what percent of the total flight time?

1. The duration of the descent is known.
2. The total flight time is known.

[Reveal] Spoiler: OA
D

Source: GMAT Club Tests - hardest GMAT questions

IMO this question is wrongly worded. By asking for the %, the answer should be sufficient if only we are able to give a number back....
therefore IMO rephrase the question or give some numbers to (1) and (2)

the question allows the answer to be different values...different values mean different answers therefore insufficient statements

I beg to differ.
DS questions do not ask us to find the answer, they just ask us whether or not it is possible to find the answer. Agreed, for different values of Descent Time and Total Time we'd get different values, but answer to DS question would always be same "Yes, we can find the answer" or "No, we can not find answer", whichever the case may be.

: ) i disagree - the DS questions ask if the info is enought to answer the question! the statements are not enough to answer the question! if the statements were sufficient to answer the question, then our answer should be a defined % !

anyone else who would like to contribute to this discusion?
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Re: ds rate (m08q34) [#permalink]

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30 Oct 2010, 18:56
Thanks for the explaination guys.
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Re: ds rate (m08q34) [#permalink]

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02 Feb 2011, 12:58
GMAT TIGER wrote:
bmwhype2 wrote:
A plane takes off from a hill at 750 meters above sea level and lands some time later in a town located at 50 meters below sea level. During the first part of its flight, the plane gained height at a rate of 50 meters per minute but then it started to descend at a rate of 20 meters per minute. The duration of the first part of the flight was what percent of the total flight time?

1. The duration of the descent is known.
2. The total flight time is known.

mandb wrote:
walker wrote:
D

general equation: 750-50=20*td-50ta ==> 2*td-5*ta=70
where ta - time of ascent, td - time of descent

1. we know td. we can find ta from general equation. SUFF.
2. we know ta+td. we can solve system of 2 equations. SUFF.

Thx... I was wondering why it was 750+50 in explanation..

It is.

Present height + (+ accent) - (- level below the sea) = 20d

750 + 50a - (-50) = 20d
75 + 5a + 5 = 2d
80 + 5a = 2d

St. 1) The duration of the descent (i.e. d) is known.
Say d = 50, a = 4. SUFF...

St. 2) The total flight (i.e. a+d) time is known.
For simplicity, say a+d = 54
a = 54 - d

Then, 80 + 5(54-d) = 2d.
d = 50. SUFF.

D

i am still confused with the above highlighted part. can somebody explain?
i cant visualize how the distance without considering slope is obtained. something like drawing would be highly appreciated.
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Re: ds rate (m08q34) [#permalink]

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02 Nov 2011, 08:25
Takeaway: Essentially this is a system of eqn or substitution problem

20y = 800 + 50x

or

20y - 50x = 800

1) we know y; sufficient
2) a+Y=given; sufficient

Try to get one variable.

Edit: oops mistype; lol i give up typing on my phone...
Posted from my mobile device

Last edited by snkrhed on 02 Nov 2011, 08:38, edited 2 times in total.
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Re: ds rate (m08q34) [#permalink]

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02 Nov 2011, 08:33
tinki wrote:
GMAT TIGER wrote:

Present height + (+ accent) - (- level below the sea) = 20d

750 + 50a - (-50) = 20d
75 + 5a + 5 = 2d
80 + 5a = 2d

St. 1) The duration of the descent (i.e. d) is known.
Say d = 50, a = 4. SUFF...

St. 2) The total flight (i.e. a+d) time is known.
For simplicity, say a+d = 54
a = 54 - d

Then, 80 + 5(54-d) = 2d.
d = 50. SUFF.

D

i am still confused with the above highlighted part. can somebody explain?
i cant visualize how the distance without considering slope is obtained. something like drawing would be highly appreciated.
[/quote]

Descent = beginning + ascend / min - ending location

Or

20y = 750+50x-(-50)

Posted from my mobile device

Posted from my mobile device
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Re: ds rate (m08q34) [#permalink]

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02 Nov 2011, 09:24
Thanks for explaining.
Re: ds rate (m08q34)   [#permalink] 02 Nov 2011, 09:24

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