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# In pentagon PQRST, PQ= 3, QR = 2, RS = 4, and ST = 5. Which

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In pentagon PQRST, PQ= 3, QR = 2, RS = 4, and ST = 5. Which [#permalink]

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11 Mar 2014, 02:17
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The Official Guide For GMAT® Quantitative Review, 2ND Edition

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Untitled.png [ 16.3 KiB | Viewed 10783 times ]
In pentagon PQRST, PQ= 3, QR = 2, RS = 4, and ST = 5. Which of the lengths 5, 10, and 15 could be the value of PT ?

(A) 5 only
(B) 15 only
(C) 5 and 10 only
(D) 10 and 15 only
(E) 5, 10, and 15

Problem Solving
Question: 150
Category: Geometry
Page: 82
Difficulty: 600

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Re: In pentagon PQRST, PQ= 3, QR = 2, RS = 4, and ST = 5. Which [#permalink]

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11 Mar 2014, 02:17
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SOLUTION

In pentagon PQRST, PQ= 3, QR = 2, RS = 4, and ST = 5. Which of the lengths 5, 10, and 15 could be the value of PT ?

(A) 5 only
(B) 15 only
(C) 5 and 10 only
(D) 10 and 15 only
(E) 5, 10, and 15

The length of any side of a triangle must smaller than the sum of the other two sides.

The same for pentagon: the length of any side of a pentagon must be smaller than the sum of the other four sides.

PQ + QR + RS + ST = 3 + 2 + 4 + 5 = 14, so the length of the fifths side can not be more than 14.

Answer: C (5 and 10 only).
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Re: In pentagon PQRST, PQ= 3, QR = 2, RS = 4, and ST = 5. Which [#permalink]

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11 Mar 2014, 22:39
4
KUDOS
The pentagon can be split in to 3 triangles: PQR, PRS and PST
Consider Triangle PQR:
Since PQ= 3, QR = 2; we can say that 1<PR<5 - Based on the property of triangles: The length of any side of a triangle must be smaller than the sum of the other 2 sides and greater than the difference of the other 2 sides.

Consider Triangle PRS:
We can say that 1<PS<9;

Consider Triangle PST:
We can say that PT is definitely less than 15. So, eliminate B, D and E.
4<PT<14;

From the answer choices, 5 and 10 hold true.

Ans is (C).

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Re: In pentagon PQRST, PQ= 3, QR = 2, RS = 4, and ST = 5. Which [#permalink]

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11 Mar 2014, 02:26
2
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Bunuel wrote:
RESERVED FOR A SOLUTION.

Divide the pentagon into triangles:

From triangle PQR, PQ+QR > PR,
or, PR < 5 ---(1)

From triangle PRS, PR + RS > PS
or, PR + 4 > PS ---(2)
from (1) and (2), PS < 9 ---(3)

From triangle PST, PS + ST > PT
or, PS + 5 > PT ---(4)
from (3) and (4), PT < 14

So, PT can not be 15. So, C is the answer.

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Re: In pentagon PQRST, PQ= 3, QR = 2, RS = 4, and ST = 5. Which [#permalink]

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14 Mar 2014, 21:45
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Since PQ + QR + RS + ST = 14, PT has to be < 14. Hence C

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Re: In pentagon PQRST, PQ= 3, QR = 2, RS = 4, and ST = 5. Which [#permalink]

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14 Aug 2014, 01:23
Bunuel wrote:
SOLUTION

In pentagon PQRST, PQ= 3, QR = 2, RS = 4, and ST = 5. Which of the lengths 5, 10, and 15 could be the value of PT ?

(A) 5 only
(B) 15 only
(C) 5 and 10 only
(D) 10 and 15 only
(E) 5, 10, and 15

The length of any side of a triangle must smaller than the sum of the other two sides.

The same for pentagon: the length of any side of a pentagon must be smaller than the sum of the other four sides.

PQ + QR + RS + ST = 3 + 2 + 4 + 5 = 14, so the length of the fifths side can not be more than 14.

Answer: C (5 and 10 only).

Bunuel, thanks for your explanation! I understand the solution and I agree, but I have one concern.

On the figure drawn we can see that
1) the direction of lines PQ, QR and RS is to the right from the point P. The sum of of these lines is only 9.
2) the direction of line ST is opposite (or to the left / back to point P).

So, if taking into account this fact, it appears that the line TP is <=9.

Is this reasoning incorrect only because it is written that the figure is drawn to scale?

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In pentagon PQRST, PQ= 3, QR = 2, RS = 4, and ST = 5. Which [#permalink]

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09 Oct 2017, 13:52
Divide pentagon into three triangles: Triangle RST, Triangle QRT, Triangle PQT
In RST, RS = 4, ST =5; RT has to be less than their sum, and greater than their difference; therefore, RT could be 8-2; just start with 8, and if it does not work, go back and try 7-2
In QRT, QR = 2, RT = 8, and QT is unknown. QT < 10; QT >6; therefore, QT = 9-7
In PQT, PQ = 3, QT = 9, PT unknown. PT<12; PT>6; therefore, 15 cannot be PT, thus eliminating answer choices B, D, and E
Suppose QT is 8; PT<11; PT>3, so it could still be 5, or 10
Suppose QT is 7; PT<10; PT>4. 10 is eliminated here, but since if QT equals 8, 10 is still possible, as with 5. Supposing RT is 2, then QT<4; if QT is 3, then PT<6; therefore, 5 has to be the answer since we used the smallest possible outcomes. Ten could be eliminated, however, under other conditions, it is permissible.

The only answer choice that was eliminated across every possible condition is 15. The conditions showed us that it can be 5 or 10.
Therefore, the answer is (C) 5 and 10 only

A helpful general rule: questions that involve ranges, work with extremes such as biggest or smallest.

Hope this helps

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Re: In pentagon PQRST, PQ= 3, QR = 2, RS = 4, and ST = 5. Which [#permalink]

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13 Dec 2017, 08:24
Bunuel wrote:
SOLUTION

In pentagon PQRST, PQ= 3, QR = 2, RS = 4, and ST = 5. Which of the lengths 5, 10, and 15 could be the value of PT ?

(A) 5 only
(B) 15 only
(C) 5 and 10 only
(D) 10 and 15 only
(E) 5, 10, and 15

The length of any side of a triangle must smaller than the sum of the other two sides.

The same for pentagon: the length of any side of a pentagon must be smaller than the sum of the other four sides.

PQ + QR + RS + ST = 3 + 2 + 4 + 5 = 14, so the length of the fifths side can not be more than 14.

Answer: C (5 and 10 only).

I followed the same logic and eliminated 15. My concern was about 5 and 10, the official solution contains quite a long and detailed explanation but should I bother? Is there any need in a question like that and dig further - proving that 10 or 5 can be the answer?

Kudos [?]: 2 [0], given: 40

Manager
Joined: 12 Nov 2016
Posts: 84

Kudos [?]: 2 [0], given: 40

Location: Kazakhstan
Concentration: Entrepreneurship, Finance
Schools: Sloan (S)
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GPA: 3.2
Re: In pentagon PQRST, PQ= 3, QR = 2, RS = 4, and ST = 5. Which [#permalink]

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13 Dec 2017, 22:29
Erjan_S wrote:
Bunuel wrote:
SOLUTION

In pentagon PQRST, PQ= 3, QR = 2, RS = 4, and ST = 5. Which of the lengths 5, 10, and 15 could be the value of PT ?

(A) 5 only
(B) 15 only
(C) 5 and 10 only
(D) 10 and 15 only
(E) 5, 10, and 15

The length of any side of a triangle must smaller than the sum of the other two sides.

The same for pentagon: the length of any side of a pentagon must be smaller than the sum of the other four sides.

PQ + QR + RS + ST = 3 + 2 + 4 + 5 = 14, so the length of the fifths side can not be more than 14.

Answer: C (5 and 10 only).

I followed the same logic and eliminated 15. My concern was about 5 and 10, the official solution contains quite a long and detailed explanation but should I bother? Is there any need in a question like that and dig further - proving that 10 or 5 can be the answer?

Bunuel I would much appreciate your comment

Kudos [?]: 2 [0], given: 40

Re: In pentagon PQRST, PQ= 3, QR = 2, RS = 4, and ST = 5. Which   [#permalink] 13 Dec 2017, 22:29
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