GMAT Knight Quant Tips

Edit (April 2021) -- I originally wrote the below in reply to a 'tip' suggesting a count-equations-count-unknowns DS strategy. I'm not sure what happened to the original post I was replying to. gmatclub has moved posts around to create this compilation thread, and my post has somehow found its way here. Since I don't ever delete things I've written, or edit posts to fix mistakes more serious than typos (I don't ever try to hide any mistakes I make, and I only ever add notes like this one clearly labeled as later edits), I'm adding this comment to explain why this post is here. I did not post it to this thread.

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It's true if you invent three random linear equations in three unknowns, you'll probably be able to solve. But GMAT questions are not random. They're constructed to test if you know how math works -- if you can identify when you can solve, and when you cannot. They test the exceptions just as often as the "rules". That's why any DS strategy based on counting equations and counting unknowns is essentially no better than random guessing.

That's empirically verifiable. If you try using "count equations, count unknowns" as a strategy on a large sample of official DS questions, at least on medium and hard questions, you only get the right answer about 1/3 of the time. On DS, when you can usually rule out a couple of answers by logic (because one statement alone is clearly insufficient, say), a strategy that answers 1/3 of your questions correctly is effectively no better than random guessing. The strategy does "work" about 2/3 of the time on easy-level questions, but getting only 2/3 of the easy questions right on the GMAT is not an acceptable hit rate for anyone aiming for a score above Q30 or so.

The best practice is to actually solve in situations like this. You'll discover very quickly whether you can solve or not. Especially when you have 3 unknowns, there are no simple rules (we have a fairly simple rule for 2 linear equations and 2 unknowns, but things are much more complicated with 3 unknowns).