Plugging In

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Tophie: Newbie | Next Rank: 10 Posts; Posts: 3; Joined: Tue Feb 21, 2012 10:27 am

Plugging In

by Tophie » Fri Mar 09, 2012 7:12 pm

I've started out my GMAT journey by using the Princeton Review's Cracking the GMAT. I know that Princeton's is probably not the greatest review book (at least I hope it's not), but it was one that my library had and I figured that the best way to start prepping was to grab a book and jump on in.

My question is this, with what they deem as "cosmic problems," those where the answer choices are not specific numbers, their advice is to always plug in. It seems like a great idea; faster, easier, etc., but it always going to work? Are there questions at the 700+ level where you have to use algebra to create equations? I can provide an example if necessary.

Thanks for the help.

T

Quote

Source: Beat The GMAT — Quantitative Reasoning | Fri Mar 09, 2012 7:12 pm

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Brent@GMATPrepNow: GMAT Instructor; Posts: 16207; Joined: Mon Dec 08, 2008 6:26 pm; Location: Vancouver, BC; Thanked: 5254 times; Followed by:1268 members; GMAT Score:770

by Brent@GMATPrepNow » Sun Mar 11, 2012 7:50 am

Tophie wrote:I've started out my GMAT journey by using the Princeton Review's Cracking the GMAT. I know that Princeton's is probably not the greatest review book (at least I hope it's not), but it was one that my library had and I figured that the best way to start prepping was to grab a book and jump on in.

My question is this, with what they deem as "cosmic problems," those where the answer choices are not specific numbers, their advice is to always plug in. It seems like a great idea; faster, easier, etc., but it always going to work? Are there questions at the 700+ level where you have to use algebra to create equations? I can provide an example if necessary.

Thanks for the help.

T

Great question.

I'm assuming that "Cosmic Problems" are the same as "Variables in the Answer Choices" questions.

The short answer is: Yes, plugging in will always work . . . if you happen to plug in the right numbers.

For example, consider this question.

Joe has J dollars and Mona has M dollars. If they combine their money, how much do they have?
A) J+M
B) JM
C) J-M
D) J/M
E) J^M

The Plug-In method tells us to choose some numbers for J and M.
Let's say that J=2 and M=2.
When J and M have these values, Joe's and Mona's combined wealth is 4 dollars.

Now we need to plug J=2 and M=2 into each answer choice, and see which one evaluates to be 4.

A) 2+2=4 (bingo)
B) (2)(2) = 4 (bingo)
C) 2-2 = 0 (nope - eliminate it)
D) 2/2 = 1 (nope - eliminate it)
E) 2^2 - 4 (bingo)

Hmmm, it looks like the correct answer is A, B or E

At this point, we need to plug in some different numbers to whittle the field down to just one correct answer. For example, if we let J=5 and M=2, we can see that only answer choice A yields a combined wealth of 7 when we plug J=5 and M=2 into the expression.

So, in this example, plugging in works, but it can take a lot of time.
The fastest way to solve these questions is the apply some reasoning and algebra.

Cheers,
Brent

Brent Hanneson - Creator of GMATPrepNow.com