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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?Is there something related to order? For one of them, order matters–but for the other, order does not matter.
What exactly is “order”? Can you give me some examples of when a question is worded as a combination and that same question tweaked so it’s a permutation question?
Oh, isn’t there a formula related to these? Do I have to memorize them? What’s the difference between the two formulas?
Will it be enough to memorize this formula? If there’s something else I need to know regarding combinations and permutations as they relate to GMAT math, what is it?
All of these questions are answered in the GMAT Pill Problem Solving Pill. Watch the video above to learn a little bit about the difference between these two rigorous concepts on GMAT math.
And what if you have this same math problem but you are told that “married couples MUST stick together.”How would you answer this question?
Well, you got it! We have a GMATPill video explanation for that as well. Check it out!
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GMAT Official Guide Quant Question Categorization (12th Edition)
Below are all the GMAT Quant questions from the Official Guide 12th edition categorized into question types.
Should you find yourself having trouble in any particular area, feel free to use this chart to help you focus in on the practice questions that you are weak in. Enjoy!
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GMAT Sentence Correction – Shift Work Equations
deepaksharma1986 wrote:skim wrote:Recently implemented “shift-work equations” based on studies of the human sleep cycle have reduced sickness, sleeping on the job, fatigue among shift workers, and have raised production efficiency in various industries.
(A) fatigue among shift workers, and have raised
(B) fatigue among shift workers, and raised
(C) and fatigue among shift workers while raising
(D) lowered fatigue among shift workers, and raised
(E) and fatigue among shift workers was lowered while raising
Why is “A” incorrect? Isn’t the parallelism correct in that option - “Have reduced” and “have raised”
Whereas in option “c” it says - “have reduced” and “while raising”Good question! So it turns out this question is actually NOT testing you on parallelism–but rather on “laundry lists.” Don’t fall for the booby trap!
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).”
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