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 30second period, the beam sweeps through 3.690Â°
(2) r = 40
I didn't understand this question
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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 Xaxis 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 Xaxis. 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 Xaxis.
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 Xaxis and it will be in Quadrant I
Combined, SUFFICIENT
Final Answer: C
GMAT assassins aren't born, they're made,
Rich
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 Xaxis 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 Xaxis. 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 Xaxis.
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 Xaxis 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.[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 Xaxis 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 Xaxis. 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 Xaxis.
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 Xaxis and it will be in Quadrant I
Combined, SUFFICIENT
Final Answer: C
GMAT assassins aren't born, they're made,
Rich
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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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.
St2: We dont know the rate; Insufficient
Answer is A
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https://www.beatthegmat.com/gprep4dst278146.html
St2: We dont know the rate; Insufficient
Answer is A
More can be found here:
https://www.beatthegmat.com/gprep4dst278146.html
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Target question: In which of the four quadrants will the beam lie 30 seconds from now ?
Statement 1: In each 30second period, the beam sweeps through 3690Â°
So, the beam will spin clockwise around the screen several times over the 30second period.
IMPORTANT: Each time the beam spins 360Â°, the beam stops at the SAME PLACE as its present location.
So, if the beam spins 3600Â°, 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 3690Â°. In other words, it spins 10 complete times PLUS an extra 90Â°
So, the beam's final resting place will be 90Â° (in a clockwise direction) from its present location. Since each quadrant spans 90Â° 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