BREAKING: Target Test Prep releases Brand New 2026 On Demand GMAT prep course

Redeem
TTP OnDemand GMAT Video Course
Featured Course

Target Test Prep GMAT OnDemand

Scott Woodbury-Stewart’s private virtual classroom — 400 hours of master-class video lessons for the GMAT Focus Edition.

  • 715+ score guarantee — highest in the industry (99th percentile)
  • 52 chapters · 1,500+ lessons · 4,000+ practice questions
  • 400 hours of video · 1,500+ instructor-led HD smartboard lessons
  • 300,000+ students accepted to Harvard, Stanford, Wharton, Booth & Sloan & more
  • 24/7 live support + weekly Zoom office hours with GMAT instructors
  • TTP AI Assist — 24/7 AI-powered virtual tutor for instant help
  • 1,200+ flashcards + AI-powered study assistant & daily calendar
  • OnDemand, LiveTeach & GMAT Bootcamp formats available
  • Also: GRE, SAT Math & Executive Assessment courses
  • MBA Admissions Consulting now available
  • 🏆 2025 EdTech Breakthrough Award: Test Prep Solution Provider of the Year
  • 200,000+ students served
  • 5-day free trial — $0 to start, no auto-billing, cancel anytime
★★★★★ 5.0 (559 reviews)
130-pt guarantee
$0 to start then $127/mo
Start Free 5-Day Trial Read 559 Reviews →

Ratios & Proportions

Expert replies
Post new topic Post Reply
Previous Topic Next Topic
User avatar
Master | Next Rank: 500 Posts
Posts: 131
Joined: Thu Feb 21, 2008 7:13 pm
Location: Canada
Thanked: 3 times
GMAT Score:590
by carllecat » Wed Mar 11, 2009 5:22 am
Ok guys, I need your help to help me with the solution provided in a book. #193 in OG 11

In a certain calculus class, the ratio of the number of math majors to the number of students who are not math majors is 2 to 5. If 2 more math majors were to enter the class, the ratio would be 1 to 2. How many students are in the class?

A) 10
B) 12
C) 21
D) 28
E) 35
Top
Join the discussion
Log in to postLogin with Googleuse password
QUOTE IN REPLY
Source: — Problem Solving |
User avatar
Master | Next Rank: 500 Posts
Posts: 131
Joined: Thu Feb 21, 2008 7:13 pm
Location: Canada
Thanked: 3 times
GMAT Score:590

by carllecat » Wed Mar 11, 2009 5:28 am
I want to know if this answer makes sense, because it does to me.

You can get the answer right in 15 seconds. The original ratio is 2 o 5 (ratio has a total of 7 parts). So the total number of students must be a multiple of 7. Cross off A and B. (MAKES SENSE TO ME)

Once we add 2 students, the ratio becomes 2 to 1 (ratio has a total of 3 parts). So if we add 2 to the total, the new number of students must be a multiple of 3. Of the remaining answer choices (21, 28 and 35), only D will get us a multiple of 3 after we add the 2 more students. (DOES NOT MAKE SENSE TO ME... ANSWER C - 21 IS ALSO A MULTIPLE OF 3 ISN'T IT?)
The book GMATfIX - The OG Companion suggests to guees answer D.
Top
Join the discussion
Log in to postLogin with Googleuse password
QUOTE IN REPLY
Master | Next Rank: 500 Posts
Posts: 353
Joined: Sat Jan 20, 2007 1:29 am
Location: Italy
Thanked: 7 times
GMAT Score:720

by mjjking » Wed Mar 11, 2009 5:56 am
So, let's call:

M -> math majors
NM -> non math majors

we have that:

M/NM = 2/5 and that if M+2 --> M+2/NM = 1/2

hence 5M = 2NM and 2M+4 = 1NM

for substitution of NM into the first equation we can solve for M and get M=8, which means NM = 20. Total students = 28.

thanks
Beat The GMAT - 1st priority
Enter a top MBA program - 2nd priority
Loving my wife: MOST IMPORTANT OF ALL!

REAL THING 1 (AUG 2007): 680 (Q43, V40)
REAL THING 2 (APR 2009): 720 (Q47, V41)
Top
Join the discussion
Log in to postLogin with Googleuse password
QUOTE IN REPLY
User avatar
Master | Next Rank: 500 Posts
Posts: 131
Joined: Thu Feb 21, 2008 7:13 pm
Location: Canada
Thanked: 3 times
GMAT Score:590

by carllecat » Wed Mar 11, 2009 6:00 am
mjjking wrote:So, let's call:

M -> math majors
NM -> non math majors

we have that:

M/NM = 2/5 and that if M+2 --> M+2/NM = 1/2

hence 5M = 2NM and 2M+4 = 1NM

for substitution of NM into the first equation we can solve for M and get M=8, which means NM = 20. Total students = 28.

thanks
Thanks for the help. However, I am looking to verify if the shortcut explanation given ir right. Does that make sense to you?
Top
Join the discussion
Log in to postLogin with Googleuse password
QUOTE IN REPLY
Master | Next Rank: 500 Posts
Posts: 139
Joined: Wed Oct 22, 2008 4:36 am
Thanked: 17 times

by sacx » Wed Mar 11, 2009 7:19 am
only D will get us a multiple of 3 after we add the 2 more students. (DOES NOT MAKE SENSE TO ME... ANSWER C - 21 IS ALSO A MULTIPLE OF 3 ISN'T IT?)

What you are missing here is that you need to add '2' before checking whether it is a multiple of 3

21 + 2 not a multiple of 3
28 + 2 multiple of 3
35 + 2 not a multiple of 3
SACX
Top
Join the discussion
Log in to postLogin with Googleuse password
QUOTE IN REPLY
User avatar
Master | Next Rank: 500 Posts
Posts: 131
Joined: Thu Feb 21, 2008 7:13 pm
Location: Canada
Thanked: 3 times
GMAT Score:590

by carllecat » Wed Mar 11, 2009 7:50 am
Thanks Sacx it is not clear to me!

What a nice shortcut it is!

Thanks again.
Top
Join the discussion
Log in to postLogin with Googleuse password
QUOTE IN REPLY
User avatar
Master | Next Rank: 500 Posts
Posts: 131
Joined: Thu Feb 21, 2008 7:13 pm
Location: Canada
Thanked: 3 times
GMAT Score:590

by carllecat » Mon Mar 16, 2009 2:02 pm
The current ratio of boys to girls at a certain school is 2to 5. If 12 additional boys were added to the school, the new ratio of boys to girls would be 4 to 9. How many boys currently attend the school?
(A) 27
(B) 48
(C) 54
(D) 72
(E) 108

For as far as I am concerned, no answer choices are multiples of 7. What am I missing here?

P.S.: I know how to solve the problem, just looking if the explained shortcup works good... and it doesn't.
Top
Join the discussion
Log in to postLogin with Googleuse password
QUOTE IN REPLY
User avatar
Master | Next Rank: 500 Posts
Posts: 131
Joined: Thu Feb 21, 2008 7:13 pm
Location: Canada
Thanked: 3 times
GMAT Score:590

by carllecat » Mon Mar 16, 2009 2:03 pm
The answer is D) 108, which is not a multiple of 7 (ratio 2 to 5).
Top
Join the discussion
Log in to postLogin with Googleuse password
QUOTE IN REPLY
Master | Next Rank: 500 Posts
Posts: 134
Joined: Sun Feb 15, 2009 7:44 pm
Thanked: 14 times

by m&m » Mon Mar 16, 2009 4:44 pm
...I would suggest sticking to the full proof method of solving. It's only 1 formula, shouldn't take longer than 1 mins to solve.

(2x+12)/5x = 4/9 cross multiply and solve for x, x=54. Number of boys is 2*54=108


to solve in the other method, you need to remember we are solving for number of BOYS, not total students. So first clue tells us that boys are a multiple of 2 (ratio of 2/5), so A is out.

Second clue tells us that by adding 12 boys are a multiple of 4. So:
b) 48+12 = 60 (yes)
c) 54+12 = 66 (no)
d) 72+12= 84 (yes)
e) 108+12=120 (yes)

now you are left with 3 potential answers. Using your method there may be infinitely many potential solutions, I would recommend using the first method.
Top
Join the discussion
Log in to postLogin with Googleuse password
QUOTE IN REPLY
Post new topic Post Reply
• Page 1 of 1