Live GMAT classes with Target Test Prep start Nov 21 and Nov 22. First class free.

See classes
Target Test Prep · GMAT

Choose how you want to prepare

Learn live with an expert or move at your own pace. Every option includes the complete TTP study system.

Newsweek Readers’ Choice Awards 2026 winner: Target Test Prep, #1 Test Prep

Target Test Prep was voted #1 Best Test Prep in the 2026 Newsweek Readers’ Choice™ Awards.

★★★★★5.0563 reviews
GMATLiveTeachStarts Nov 22
Josh Braslow, Target Test Prep GMAT expert
LIVE ONLINE CLASSES

Get Ready for GMAT Test Day Faster with Live Online Classes

with Josh Braslow, 20+ years in test prep

Nov 22 · Josh Braslow
Sun · 9:30 AM to 12:30 PM ET
Included
40 hours of live online classes + 6 months of TTP OnDemand
  • Attend the first class for free
  • Every class is recorded, so you never fall behind
View classes & enroll
Limited seats availableTarget Test Prep
EALiveTeachOnDemandStart anytime
EXECUTIVE ASSESSMENT

Target Test Prep EA OnDemand

Self-paced EA prep. Study on your schedule.

EXECUTIVE ASSESSMENT

with

165+ EA score guarantee
$05-day trial no automatic billing
Schedule
Included
40 hours of live online classes plus six months of access to the complete TTP EA OnDemand course.
  • 165+ EA Score Guarantee
  • 4,100+ Quant, Verbal, and Integrated Reasoning practice questions
  • 400+ hours of in-depth video lessons
  • 3,000+ step-by-step video solutions
View EA class & enroll Start free 5-day trial
Limited cohort · enrollment openTrial includes full course accessTarget Test Prep
GMATOnDemandStart anytime
SELF-PACED MASTERCLASS

Target Test Prep GMAT OnDemand

Complete access from day one. Study on your schedule.

715+ score guarantee
$0to start then $127/mo
  • Personalized study plan and analytics
  • Thousands of lessons and practice questions
Start free 5-day trial
Read 563 reviewsTarget Test Prep

When to subtract?

Problem Solving — algebra and arithmetic
Expert replies
Post new topic Post Reply
Subscribe topic
Previous Topic Next Topic
Senior | Next Rank: 100 Posts
Posts: 42
Joined: Thu Sep 11, 2008 11:07 pm
Location: California
Thanked: 3 times
by cmr209 » Wed Sep 09, 2009 2:44 pm
With the following problem:
m + 2n = 13 and 3m + 2n = 21

How do I know to add or subtract the two equations?
The word "and" generally tells me to add, yet in this case it appears that they need to be subtracted.
Top
Join the discussion
Source: — Quantitative Reasoning |
Master | Next Rank: 500 Posts
Posts: 159
Joined: Thu Aug 27, 2009 10:30 am
Thanked: 19 times

by bharathh » Wed Sep 09, 2009 3:02 pm
Depends on which term you are solving for. You want to subtract to get rid of a particular term.

In this case it is easy to get rid of all the n terms by subtracting the equations. Solve for m and then plug in the value of m to find n.
Top
Join the discussion
User avatar
GMAT Instructor
Posts: 80
Joined: Wed Aug 12, 2009 8:49 pm
Location: San Francisco, CA
Thanked: 16 times

by grockit_jake » Sun Sep 13, 2009 9:00 am
Adding and subtracting equations is important and good to keep in mind, but there is no rule.

It depends what variables the question is asking for, and in what capacity.

In your given set:


m + 2n = 13
3m + 2n = 21

You can see that since the n's have the same coefficient, you can subtract and be left with m's only. If you add, you will see nothing will drop out.

Also, be careful not to screw up your signs if you subtract in the wrong direction ( -2m = -8 OR 2m = 8)
Jake Becker
Academic Director
Grockit Test Prep
https://www.grockit.com
Top
Join the discussion