One equation,...two variables?? : Quant Question Archive [LOCKED]
Check GMAT Club Decision Tracker for the Latest School Decision Releases http://gmatclub.com/AppTrack

 It is currently 19 Jan 2017, 19:58

### GMAT Club Daily Prep

#### Thank you for using the timer - this advanced tool can estimate your performance and suggest more practice questions. We have subscribed you to Daily Prep Questions via email.

Customized
for You

we will pick new questions that match your level based on your Timer History

Track

every week, we’ll send you an estimated GMAT score based on your performance

Practice
Pays

we will pick new questions that match your level based on your Timer History

# Events & Promotions

###### Events & Promotions in June
Open Detailed Calendar

# One equation,...two variables??

Author Message
Intern
Joined: 26 Aug 2009
Posts: 22
Followers: 0

Kudos [?]: 4 [1] , given: 3

### Show Tags

10 Sep 2009, 08:08
1
KUDOS
This topic is locked. If you want to discuss this question please re-post it in the respective forum.

I curious,... I thought that in order to solve an equation (or in the case of a D.S. question - in order to know if one possibility is "SUFF"), that an equation must have only one variable.

In other words, if we are given the equation:

2a + 5b = 20

then we can't solve it without any other info. But if we try to simplify it down to finding "a" then we get:

2a = 20 - 5b or a = 10 - (5/2)b

If we try to plug in the equation again we get:

2[10 - (5/2)b] + 5b = 20 or 20 - 5b + 5b = 20 or

20 = 20

which leaves us with nothing.

But if a question has two equations, then we could use this plug-n-play approach to find the variables.

So, after trying to tackle the attached Kaplan D.S. question, I'm left wondering..... In both possibilities, it states that there is one equation, two variables but the first possibility states that we move to the second and then, all of a sudden, it's possible to solve one equation, two variables ??

What's going on here?
Attachments

ScreenHunter_02 Sep. 10 17.55.gif [ 19.41 KiB | Viewed 32419 times ]

 Kaplan GMAT Prep Discount Codes Veritas Prep GMAT Discount Codes e-GMAT Discount Codes
Kaplan GMAT Instructor
Joined: 25 Aug 2009
Posts: 644
Location: Cambridge, MA
Followers: 83

Kudos [?]: 276 [1] , given: 2

### Show Tags

10 Sep 2009, 09:08
1
KUDOS
Expert's post
Hi Stilite,

You're slightly misremembering the rule, though it's understandable as the distinction is subtle.. The rule is that in order to solve for all variables in a system of equations, we need at least as many equations as we have variables. It is mathematically impossible for a single equation to give us values for both a and b. However, since we only need the value of one variable in this problem, b, it is possible for a single equation to give us a solution; we just need an equation where we can get rid of all the A's.

In this case, because we have +a on both sides, the a's cancel out--leaving us with a single variable equation with nothing but b's and numbers. This is a common trap in DS questions. Keep your eyes peeled for variables that aren't really there!
_________________

Eli Meyer
Kaplan Teacher
http://www.kaptest.com/GMAT

Prepare with Kaplan and save \$150 on a course!

Kaplan Reviews

Intern
Joined: 26 Aug 2009
Posts: 22
Followers: 0

Kudos [?]: 4 [0], given: 3

### Show Tags

11 Sep 2009, 06:52
KapTeacherEli wrote:
Hi Stilite,

You're slightly misremembering the rule, though it's understandable as the distinction is subtle.. The rule is that in order to solve for all variables in a system of equations, we need at least as many equations as we have variables. It is mathematically impossible for a single equation to give us values for both a and b. However, since we only need the value of one variable in this problem, b, it is possible for a single equation to give us a solution; we just need an equation where we can get rid of all the A's.

In this case, because we have +a on both sides, the a's cancel out--leaving us with a single variable equation with nothing but b's and numbers. This is a common trap in DS questions. Keep your eyes peeled for variables that aren't really there!

Probably the best/clearest response I've ever received. Many thanks for that!
Re: One equation,...two variables??   [#permalink] 11 Sep 2009, 06:52
Display posts from previous: Sort by