I didn't understand this question

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I didn't understand this question

by phanikpk » Sat Jul 12, 2014 5:37 pm
The figure above shows the present position on a radar screen of a sweeping beam that is rotating at a constant rate in a clockwise direction. In which of the four quadrants will the beam lie 30 seconds from now ?
(1) In each 30-second period, the beam sweeps through 3.690°
(2) r = 40

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by [email protected] » Sat Jul 12, 2014 5:48 pm
Hi phanikpk,

In DS questions, you have to be suspicious of any drawings that are included (because sometimes pictures are not drawn to scale).

In this prompt, the language states the "present location" of a "sweeping beam" (the radius) as it moves clockwise around the circle. The question asks what quadrant of the graph the beam will be in in 30 seconds.

To answer this question, it looks like we'll need to know the starting position of the beam AND its rate of movement.

From the drawing and the wording, we know that the beam is currently in Quadrant I, but we don't know exactly where.

Fact 1: Every 30 seconds, the beam moves 3.69 degrees.

Depending on where the beam actually begins, it might still be in Quadrant I in 30 seconds.
If the beam starts really close to the X-axis though, it might move into Quadrant 4 in 30 seconds.
Fact 1 is INSUFFICIENT

Fact 2: R = 40

This tells us the angle that the beam is from the X-axis. However, without knowing what rate the beam is moving, it could be in any of the 4 Quadrants in 30 seconds.
Fact 2 is INSUFFICIENT

Combined,
We know that the beam is 40 degrees off of the X-axis.
We know that the beam moves 3.69 degrees clockwise every 30 seconds.

So...in 30 seconds, the beam will be 36.31 degrees off of the X-axis and it will be in Quadrant I
Combined, SUFFICIENT

Final Answer: C

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by phanikpk » Sat Jul 12, 2014 5:51 pm
[email protected] wrote:Hi phanikpk,

In DS questions, you have to be suspicious of any drawings that are included (because sometimes pictures are not drawn to scale).

In this prompt, the language states the "present location" of a "sweeping beam" (the radius) as it moves clockwise around the circle. The question asks what quadrant of the graph the beam will be in in 30 seconds.

To answer this question, it looks like we'll need to know the starting position of the beam AND its rate of movement.

From the drawing and the wording, we know that the beam is currently in Quadrant I, but we don't know exactly where.

Fact 1: Every 30 seconds, the beam moves 3.69 degrees.

Depending on where the beam actually begins, it might still be in Quadrant I in 30 seconds.
If the beam starts really close to the X-axis though, it might move into Quadrant 4 in 30 seconds.
Fact 1 is INSUFFICIENT

Fact 2: R = 40

This tells us the angle that the beam is from the X-axis. However, without knowing what rate the beam is moving, it could be in any of the 4 Quadrants in 30 seconds.
Fact 2 is INSUFFICIENT

Combined,
We know that the beam is 40 degrees off of the X-axis.
We know that the beam moves 3.69 degrees clockwise every 30 seconds.

So...in 30 seconds, the beam will be 36.31 degrees off of the X-axis and it will be in Quadrant I
Combined, SUFFICIENT

Final Answer: C

GMAT assassins aren't born, they're made,
Rich
Thanks rich for a beautiful explanation.
My doubt came whether I have to rely on the image (given in the qn) to determine the present location of beam . So, I couldn't get anything about present location. So, I kept E.

Sure, will rely on the pic also from now on wards from some info.

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by mevicks » Sat Jun 11, 2016 7:52 pm
St1: 30 Second movement is provided thus we can find the final quadrant = 4th. Even if we consider extremes of 1st quadrant (1 degree or 89 degrees) the resultant quadrant will still be 4. Sufficient.

Image

St2: We dont know the rate; Insufficient

Answer is A


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by [email protected] » Sun Jun 12, 2016 6:37 am
Image

The figure above shows the present position on a radar screen of a sweeping beam that is rotating at a constant rate in a clockwise direction. In which of the four quadrants will the beam lie 30 seconds from now ?

(1) In each 30-second period, the beam sweeps through 3690°
(2) r = 40
Target question: In which of the four quadrants will the beam lie 30 seconds from now ?

Statement 1: In each 30-second period, the beam sweeps through 3690°
So, the beam will spin clockwise around the screen several times over the 30-second period.

IMPORTANT: Each time the beam spins 360°, the beam stops at the SAME PLACE as its present location.
So, if the beam spins 3600°, it is spinning around 10 times, and stopping at the SAME PLACE as its present location. At the moment, the beam is is Quadrant I.

Statement 1 says that the beam spins 3690°. In other words, it spins 10 complete times PLUS an extra 90°
So, the beam's final resting place will be 90° (in a clockwise direction) from its present location. Since each quadrant spans 90° and since the beam is presently in Quadrant I, we can be certain that the beam will end up in Quadrant IV
Since we can answer the target question with certainty, statement 1 is SUFFICIENT

Statement 2: r = 40
No information about how the beam will move during the next 30 seconds.
Since we cannot answer the target question with certainty, statement 2 is NOT SUFFICIENT

Answer = A

Cheers,
Brent
Brent Hanneson - Creator of GMATPrepNow.com
Image