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In the figure shown above, line segment QR has length 12. [#permalink]

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05 Jan 2014, 16:49

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In the figure shown above, line segment QR has length 12, and rectangle MPQT is a square. If the area of rectangular region MPRS is 540, what is the area of rectangular region TQRS?

In the figure shown above, line segment QR has length 12, and rectangle MPQT is a square. If the area of rectangular region MPRS is 540, what is the area of rectangular region TQRS?

(A) 144 (B) 216 (C) 324 (D) 360 (E) 396

The area of MPRS = the area of MPQT + the area of TQRS.

Re: In the figure shown above, line segment QR has length 12. [#permalink]

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06 Jan 2014, 00:12

oss198 wrote:

Bunuel wrote:

The area of MPRS = the area of MPQT + the area of TQRS.

540 = x^2 + 12x --> x = 18.

The area of TQRS = 12*18 = 216.

Answer: B.

Hi Bunuel, thank you for the answer, can you explain your technique to find the x quickly? It takes me a lot of time..

If I may,

To find the roots of the equation ax^2+bx+c=0 by the formulae

This equation has 2 solutions given by (-b+/-\(\sqrt{b^2-4ac}\))/2a------>

(-12+/-\(\sqrt{12^2-4*540}\))/2 So Roots are (-12 +/-48 )/2------> -30 and 18 since length cannot be negative so X=18 and therefore the area =18*12----->216
_________________

“If you can't fly then run, if you can't run then walk, if you can't walk then crawl, but whatever you do you have to keep moving forward.”

Re: In the figure shown above, line segment QR has length 12. [#permalink]

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06 Jan 2014, 00:22

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oss198 wrote:

Attachment:

Picture 1.png

In the figure shown above, line segment QR has length 12, and rectangle MPQT is a square. If the area of rectangular region MPRS is 540, what is the area of rectangular region TQRS?

(A) 144 (B) 216 (C) 324 (D) 360 (E) 396

A very good contender for back solving, lets start with option C (Always start with C)

Area of rectangle.........Side QT.......Area of square.....Total area of the figure (A) 144 (B) 216 (C) 324....................324/12=27.... 27*27 =729........... 729 + 324 = 1053 (D) 360 (E) 396

The total area was supposed to be 540 however it is 1053 hence we move up (go for smaller values). Now we know that answer is either A or B so we try any of them.

Area of rectangle.........Side QT.......Area of square.....Total area of the figure (A) 144 (B) 216....................216/12 =18.....18*18 =324.......... 324 + 216 =540 (BINGO!) (C) 324....................324/12=27.... 27*27 =729........... 729 + 324 = 1053 (D) 360 (E) 396

Answer is B.
_________________

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Re: In the figure shown above, line segment QR has length 12. [#permalink]

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06 Jan 2014, 16:42

WoundedTiger wrote:

oss198 wrote:

Bunuel wrote:

The area of MPRS = the area of MPQT + the area of TQRS.

540 = x^2 + 12x --> x = 18.

The area of TQRS = 12*18 = 216.

Answer: B.

Hi Bunuel, thank you for the answer, can you explain your technique to find the x quickly? It takes me a lot of time..

If I may,

To find the roots of the equation ax^2+bx+c=0 by the formulae

This equation has 2 solutions given by (-b+/-\(\sqrt{b^2-4ac}\))/2a------>

(-12+/-\(\sqrt{12^2-4*540}\))/2 So Roots are (-12 +/-48 )/2------> -30 and 18 since length cannot be negative so X=18 and therefore the area =18*12----->216

Thank you for your answer, but still : how do you find 48 quickly? you don't find the root of 12^2-4*540 so easily, do you?

Thank you for your answer, but still : how do you find 48 quickly? you don't find the root of 12^2-4*540 so easily, do you?

ps : your signature is awesome

It's easy to solve this equation without using the formula (which is cumbersome in my opinion) x^2 + 12x - 540 = 0

We try to split 540 into two factors such that their difference is 12 540 = 54*10 = 18*3*10 You can see immediately that the 2 factors will be 18 and 30.

Re: In the figure shown above, line segment QR has length 12. [#permalink]

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23 Feb 2014, 22:06

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Factorize 540 & pick up the best fit 540 = 2 x 270 = 2 x 2 x 135 = 2 x 2 x 3 x 45 = 2 x 2 x 3 x 3 x 3 x 5

Rearranging: = (2 x 3 x 5) x (2 x 3 x 3) No. in above brackets add upto as 30 & 18; there multiplication is 540 & substraction is 12, so they best fit in the equation
_________________

Re: In the figure shown above, line segment QR has length 12. [#permalink]

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17 Jul 2014, 16:55

VeritasPrepKarishma wrote:

oss198 wrote:

Thank you for your answer, but still : how do you find 48 quickly? you don't find the root of 12^2-4*540 so easily, do you?

ps : your signature is awesome

It's easy to solve this equation without using the formula (which is cumbersome in my opinion) x^2 + 12x - 540 = 0

We try to split 540 into two factors such that their difference is 12 540 = 54*10 = 18*3*10 You can see immediately that the 2 factors will be 18 and 30.

Would you say though that this technique would be even faster than simply back-solving when it comes to quadratic equations?

Thanks

Franco

Backsolving is a useful technique but its relevance in GMAT is decreasing. The options given are such that it is harder to back solve now. For example, here you will need to divide the option by 12 (the length of QR) and then try to plug in what you get in the equation. Notice that all options are divisible by 12 (which will be true for all good options) so it might be some time before you arrive at the answer using backsolving. During the exam, use whatever comes to your mind - you wouldn't have the time to consciously decide to use one method over another.
_________________

I guess my problem is just the high number of 540 but unfortunately the GMAT often uses such high number in quadratic equations related to these kind of geometric problems
_________________

Re: In the figure shown above, line segment QR has length 12. [#permalink]

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25 May 2017, 10:35

1

This post was BOOKMARKED

oss198 wrote:

Attachment:

Picture 1.png

In the figure shown above, line segment QR has length 12, and rectangle MPQT is a square. If the area of rectangular region MPRS is 540, what is the area of rectangular region TQRS?

(A) 144 (B) 216 (C) 324 (D) 360 (E) 396

First re-draw the image on the page and replace QR with 12 and PQ with x. We know that PM is also x, so the total area is (12+x)*x = 540. It turns into the quadratic 12x + x^2 = 540 or -540 + x^2 + 12x. The toughest part of the problem is figuring out which factor roots to use. The best method is to realize 540 is 9*60 and double 9 while halving 60 to get 18*30. These are separated by 12, the distance we need for the middle term of the quadratic (12x).

In the figure shown above, line segment QR has length 12. [#permalink]

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05 Jul 2017, 21:45

I was scratching my head when I saw 540 but when I wrote it as 54*10 and then 9*6*5*2, a light bulb lit up. 18 and 30. Now looking at the LHS to get 12 we need a larger positive number. This means 18 is the only option that stays non negative when factored.
_________________

Kudosity killed the cat but your kudos can save it.

Re: In the figure shown above, line segment QR has length 12. [#permalink]

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02 Oct 2017, 09:54

PerfectScores wrote:

oss198 wrote:

Attachment:

Picture 1.png

In the figure shown above, line segment QR has length 12, and rectangle MPQT is a square. If the area of rectangular region MPRS is 540, what is the area of rectangular region TQRS?

(A) 144 (B) 216 (C) 324 (D) 360 (E) 396

A very good contender for back solving, lets start with option C (Always start with C)

Area of rectangle.........Side QT.......Area of square.....Total area of the figure (A) 144 (B) 216 (C) 324....................324/12=27.... 27*27 =729........... 729 + 324 = 1053 (D) 360 (E) 396

The total area was supposed to be 540 however it is 1053 hence we move up (go for smaller values). Now we know that answer is either A or B so we try any of them.

Area of rectangle.........Side QT.......Area of square.....Total area of the figure (A) 144 (B) 216....................216/12 =18.....18*18 =324.......... 324 + 216 =540 (BINGO!) (C) 324....................324/12=27.... 27*27 =729........... 729 + 324 = 1053 (D) 360 (E) 396

Answer is B.

why do you always start with C? Also can you please explain your strategy? Thanks.

In the figure shown above, line segment QR has length 12. [#permalink]

Show Tags

07 Oct 2017, 09:43

oss198 wrote:

Attachment:

Picture 1.png

In the figure shown above, line segment QR has length 12, and rectangle MPQT is a square. If the area of rectangular region MPRS is 540, what is the area of rectangular region TQRS?

(A) 144 (B) 216 (C) 324 (D) 360 (E) 396

Solved it using quadratic expression. (12 + x) = length = PR x = length of PQ MP = PQ because square therefore, no need for second variable, just use x again (12 + x) (x) = 540 x^2 + 12x - 540 = 0 (x - 18 ) (x + 30) = 0 x = 18, -30

In the figure shown above, line segment QR has length 12. [#permalink]

Show Tags

07 Oct 2017, 14:07

ayas7 wrote:

PerfectScores wrote:

oss198 wrote:

Attachment:

Picture 1.png

In the figure shown above, line segment QR has length 12, and rectangle MPQT is a square. If the area of rectangular region MPRS is 540, what is the area of rectangular region TQRS?

(A) 144 (B) 216 (C) 324 (D) 360 (E) 396

A very good contender for back solving, lets start with option C (Always start with C)

Area of rectangle.........Side QT.......Area of square.....Total area of the figure (A) 144 (B) 216 (C) 324....................324/12=27.... 27*27 =729........... 729 + 324 = 1053 (D) 360 (E) 396

The total area was supposed to be 540 however it is 1053 hence we move up (go for smaller values). Now we know that answer is either A or B so we try any of them.

Area of rectangle.........Side QT.......Area of square.....Total area of the figure (A) 144 (B) 216....................216/12 =18.....18*18 =324.......... 324 + 216 =540 (BINGO!) (C) 324....................324/12=27.... 27*27 =729........... 729 + 324 = 1053 (D) 360 (E) 396

Answer is B.

why do you always start with C? Also can you please explain your strategy? Thanks.

For questions where the answer choices are listed in ascending order, and we want to backsolve, we start with C because it is the middle value. C gives a benchmark.

If C yields an answer that is too large? Toss out Answers D and E, which will be greater than C. Then we only have to pick between Answers A and B.

There are two really good explanations for starting with C, given by a GMATclub expert, HERE and HERE

THIS POST by Bunuel includes the link I gave above, and a lot more. Scroll down slightly to "2. Strategies and Tactics."

One more. The post immediately above, which I just found rather accidentally, is part of what looks to be a phenomenal collection, Ultimate GMAT Quantitative Megathread, by Bunuel, composed of GMAT Quant . . . everything. MEGATHREAD HERE

There are two really good explanations for starting with C, given by a GMATclub expert, HERE and HERE

THIS POST by Bunuel includes the link I gave above, and a lot more. Scroll down slightly to "2. Strategies and Tactics."

One more. The post immediately above, which I just found rather accidentally, is part of what looks to be a phenomenal collection, Ultimate GMAT Quantitative Megathread, by Bunuel, composed of GMAT Quant . . . everything. MEGATHREAD HERE