Problem Solving- Backsolving strategies

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aanderson04: Junior | Next Rank: 30 Posts; Posts: 20; Joined: Fri Jul 17, 2009 4:41 pm; Thanked: 3 times

by aanderson04 » Sat Jul 18, 2009 8:03 pm

So I have studied the Kaplan strategies for backsolving on Problem Solving problems (try answer choices B or D first), but I have also printed out the flash-cards from the www.facebook.com/beatthegmat site. One of the flashcards says to begin with answer choice E first. What is the best way and why?

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Source: Beat The GMAT — GMAT Strategy | Sat Jul 18, 2009 8:03 pm

VP_Jim: GMAT Instructor; Posts: 1223; Joined: Thu May 01, 2008 3:29 pmFollowed by: 15 members; Location: Los Angeles, CA; Thanked: 185 times

by VP_Jim » Mon Jul 20, 2009 9:05 am

There are lots of opinions on this, but mine is to start with answer choice C - that is, right in the middle.

Let's say that you backsolve C and find that it's too big. Then you can eliminate A and B along with it. Now you only have D and E left.

This way, you only have to backsolve one more time. If D is wrong, then E is right, and vice versa.

If you start with E, I can't really see how that would help. If E is too big, you haven't eliminated anything along with it since A-D are all smaller. Starting with B or D makes more sense, but as I said, I do C.

Jim S. | GMAT Instructor | Veritas Prep

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Ian Stewart: GMAT Instructor; Posts: 2623; Joined: Mon Jun 02, 2008 3:17 amFollowed by: 355 members; Location: Montreal; Thanked: 1090 times; GMAT Score:780

by Ian Stewart » Mon Jul 20, 2009 9:25 am

If you'll know, after testing an answer choice and discovering it's wrong, whether the right answer should be larger or smaller, you should start with B or D. Say you start with B:

-if B is right, you're done;
-if B is too large, the answer is A, and you're done;
-if B is too small, move on to D. If D is right, you're done. If D is too large, then the answer is C and you're done. If D is too small, then the answer is E and you're done.

You only ever need to do at most two tests, and 2/5ths of the time you only need to do one test. If you start with C, you also only ever need to do two tests, but you'll always need to do a second test when C is the wrong answer - you can only get away with one test 1/5th of the time.

All of that said, knowing this is not likely to make much of a difference on your test. There aren't all that many questions you can backsolve in the first place, and fewer still on which you'll know, after testing a wrong answer, whether the correct answer is larger or smaller.

You may also read that, on questions which begin "which of the following...", it's best to start from answer choice E -- the idea being that the GMAT question designers assume people will start from A, and want to force people to do more work by trying out more answer choices. I've looked at as many official questions as I can find in this format, and there was no statistically significant difference in the frequency of each answer choice (C was actually the most common, though not by much), so I don't put much stock in that advice.

For online GMAT math tutoring, or to buy my higher-level Quant books and problem sets, contact me at ianstewartgmat at gmail.com

ianstewartgmat.com

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navalpike: Master | Next Rank: 500 Posts; Posts: 103; Joined: Mon May 04, 2009 11:53 am; Thanked: 6 times

by navalpike » Mon Jul 20, 2009 10:45 am

Great advice. thanks.

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