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Bunuel
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I ended up having to resort to plugging in the values to find a possible answer to this.

But, initially, it was important to figure out the average exposure per image on each night. This helped eliminate a lot of combinations and made my work easier.

On night 1:
x=number of faint targets & y=number of bright targets
which means, xR + yS = 625 ----- (1)
and with a total of 35 images, x + y = 35 ------ (2)

On night 2:
a=number of faint targets & b=number of bright targets
therefore, aR + bS = 1730 ----- (3)
& a + b = 140 ----- (4)

On night 1, with a total of 625 seconds for 35 images, the average exposure per image was 17.86 seconds. For night 2, with 1730 seconds for 140 images, it was 12.35 seconds per image.

With this information, we can identify the R & S pairs. Both averages have to be between the R & S values. With this condition, we can eliminate most possibilities, barring: (R,S) = (19,10), (21,10), (19,12) & (21,12)

Plugging R=19 & S=19 and using equations (1) & (2) in night 1, I couldn't get integer values for x & y. So that one's eliminated.

Then I tried R=21 & S=10 using (1) & (2), I got x=25 & y=10.
Tried the same R & S values for (3) & (4) and I got a=30 & b=110.

So, my pick is R=21 and S=10.

I would have never figured this out within 3 mins though.
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MY strategy is to skip this question as this will take 10 mins to solve by any method.
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Parag22jain
MY strategy is to skip this question as this will take 10 mins to solve by any method.
Parag22jain

Indeed, it is a good strategy for almost everyone to skip some questions on DI if they look particularly long and hard, and remember that we can always come back to them if we have extra time.

It's essential to make sure we finish and get all of the lower hanging fruit, since we are penalized much more for missing easier questions. Students often don't realize that the difficulty fluctuates a lot throughout the test, even when you're scoring very high, so you can often get some questions at the end that aren't that hard. As an example, my data from a DI 90 practice test is attached; I got 2 easy, 9 medium, and 9 hard questions.

Long and hard two-part analysis questions are often good candidates to skip.


Statistically, this question is harder than any official focus edition practice test question, with only 21% accuracy.

I was curious, so I looked at the 29 805+ TPA practice test questions; only one was below 30% accuracy, at 28%: https://gmatclub.com/forum/a-city-counc ... 26193.html

The others are all in the 30's, 40's, or 50's for % accuracy.


The advanced search here is a very useful tool: https://gmatclub.com/forum/advanced-search/

For practice questions, I recommend the "Official Guide" and "GMAT Prep (Classic)" tags for the source.
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