Need Clarification on a DS question

Expert replies

ben wade: Senior | Next Rank: 100 Posts; Posts: 32; Joined: Sun Jun 14, 2009 4:54 am; Thanked: 1 times

by ben wade » Sun Jul 19, 2009 7:12 am

If the ratio of the number of teachers to the number of students is the same in School district M and school district P, what is the ratio of the number of students in School district M to the number of students in the school district P?

(1) There are 10.000 more students in school district M than there are in school district P.
(2) The ratio of the number of teachers to the number of students in school district M is 1 to 20.

I chose answer choice as B.
My reasoning was like this -
Given - Tm/Sm = 1/20
We know that - Tm/Sm = Tp/Sp = 1/20.
Altering Tm/Sm = Tp/Sp => Sm/Sp = Tm/Tp = 1/20.

Can anyone point out the error in making this alteration in the ratios?

Source: Beat The GMAT — Data Sufficiency | Sun Jul 19, 2009 7:12 am

Stuart@KaplanGMAT: GMAT Instructor; Posts: 3224; Joined: Tue Jan 08, 2008 2:40 pmFollowed by: 614 members; Location: Toronto; Thanked: 1710 times; GMAT Score:800

Re: Need Clarification on a DS question

by Stuart@KaplanGMAT » Sun Jul 19, 2009 9:38 am

ben wade wrote:Altering Tm/Sm = Tp/Sp => Sm/Sp = Tm/Tp = 1/20.

Can anyone point out the error in making this alteration in the ratios?

You can only substitute things that are identical. You've substituted things that are different.

Using numbers to illustrate:

1/2 = 3/6 = 5/10

You rearraged the first two parts (correctly) to get:

1/2 = 3/6

2/6 = 1/3

then you just added the third part to get the equation:

2/6 = 1/3 = 5/10

which is clearly not true.

Stuart Kovinsky | Kaplan GMAT Faculty | Toronto

Kaplan Exclusive: The Official Test Day Experience | Ready to Take a Free Practice Test? | Kaplan/Beat the GMAT Member Discount
BTG100 for $100 off a full course

Umar82: Junior | Next Rank: 30 Posts; Posts: 21; Joined: Fri Nov 21, 2008 9:41 am; Thanked: 1 times

by Umar82 » Tue Aug 04, 2009 11:09 am

What's the OA . . . C?

vikram_k51: Master | Next Rank: 500 Posts; Posts: 208; Joined: Sat Jan 31, 2009 11:32 am; Location: Mumbai; Thanked: 2 times

by vikram_k51 » Tue Aug 04, 2009 11:25 am

I think it is E.There is no clear co-relation b/w the ratios.

Featured schoolRound 2 closes Jan 7, 2027 58 days left

Chicago Booth

Since 1982, 10 Booth faculty members have received the Nobel Prize in Economic Sciences, more than any other business school.

#3
U.S. News rank: $175,000
Median base salary: 670
Average GMAT Focus: 635
Students in the class

Apply to Booth Visit chicagobooth.edu See the full profile

Photo: Warren LeMay, CC BY-SA 2.0

GMAT Course Reviews

563 reviews

1168 reviews

431 reviews

359 reviews

147 reviews

5/5

5 Star (563 Reviews)

"Target Test Prep is the closest to the official version of the GMAT exam, about 99% accuracy in terms of the quality and quantity of information. The course has excellently created singular sets of focused lessons and tests for every possible topic that one could come across in the official GMAT exam."

"The TTP course maximizes the efficiency of the time you spend studying. It will take time and effort but I could almost guarantee that if you complete the course exactly as it is laid out you will get an amazing score. They also have a very responsive team willing to help with any questions you might have."

"TTP has two things that I think no other test prep company offers: A teaching approach that reinforces understanding and an attitude that will give you the mental preparedness needed to succeed on the test. TTP gives you a deep understanding of the concept you need to know while teaching you how to think."

Write a Review