Combinations Vs Permutations: What’s the Difference on GMAT Math?

So what’s the difference between these two nebulous concepts that generally appear on the most difficult of GMAT quant questions?

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GMAT Official Guide Quant Question Categorization (12th Edition)

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GMAT Sentence Correction – Shift Work Equations

Step 1) Recognize the sentence structure. Right now the sentence is structured like this:

“studies have reduced X, Y, Z, and have raised Q.”

You cannot have this structure! With a “laundry list” like this, you have to end the third item with “and…”
It has to be:
“studies have reduced X, Y, and Z while raising Q.”

I was spent a few seconds onsidering another alternate structure like this:
“studies have reduced X, Y, Z, and Q”—but that’s not what the sentence is trying to say. The sentence is trying to stay that it RAISED Q—instead of REDUCED Q. So after a few seconds, I went back to what I was thinking of before.

Step 2) So we conclude we want to REDUCE 3 items, and RAISE the last one. 

so think:

“studies have reduced item1, item2, and item3 while raising item4.”

The word “and” must be there–only choices (C) and (E) have this. Of the two, (C) is much simpler and still accurate. The “was lowered” in (E) shouldn’t be there.

Step 3) So we go with (C) and read it again to make sure it makes sense.
“studies have reduced [ (sickness), (sleeping on the job), (and fatigue among shift workers) ] while raising (production efficiency).”

More GMAT Questions Explained By GMATPill

How To Go Through GMAT Quetions

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