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

Redeem

GMAT Review PS #188 Why didn't Plugging In work?

This topic has expert replies
Post new topic Post Reply
Previous Topic Next Topic
Newbie | Next Rank: 10 Posts
Posts: 1
Joined: Thu Aug 26, 2010 12:30 pm

GMAT Review PS #188 Why didn't Plugging In work?

by nickknock » Sun Aug 29, 2010 8:07 pm
Greetings all. Here is the problem I have a question on, from the GMAT review 11th edition:

Drum X is 1/2 full of oil and drum Y, which has twice the capacity of drum X, is 2/3 full of oil. If all of the oil in drum X is poured into drum Y, then drum Y will be filled to what fraction of its capacity?

A) 3/4
B) 5/6
C) 11/12
D) 7/6
E) 11/6


MY solution was to Plug In.

Capacity of Drum X=12
Capacity of Drum Y=24
Drum X is 6/12 full
Drum Y is 16/24 full

Then I added 6/12+16/24 to get 7/6 which is wrong.

The book explained the right answer, which I understood (they left it in variables) but I don't quite understand why Plugging In did not work, can you guys help me?
Top
Source: — Problem Solving |
User avatar
Legendary Member
Posts: 1172
Joined: Wed Apr 28, 2010 6:20 pm
Thanked: 74 times
Followed by:4 members

by uwhusky » Sun Aug 29, 2010 8:15 pm
The problem with the approach is that you converted both to 24 when you add them, so they end up with the same capacity.

What you should have done is simply add 6 to to 16/24, which would be 22/24, and deduce down to 11/12, which is the correct answer. But that's probably more confusing than using the suggested method to solve the question.
Top

GMAT/MBA Expert

User avatar
GMAT Instructor
Posts: 1179
Joined: Sun Apr 11, 2010 9:07 pm
Location: Milpitas, CA
Thanked: 447 times
Followed by:88 members

by Rahul@gurome » Sun Aug 29, 2010 8:32 pm
Solution:
Let drum X's capacity be V.
So volume of oil in drum X is (1/2)*V.
Drum Y's capacity is 2V.
So volume of oil in drum Y is (2/3)*2V = (4/3)*V.
If all of the oil in drum X is poured into drum Y, volume of oil in drum Y is (1/2)*V + (4/3)*V = (11/6)*V.
Or the fraction of oil in drum Y is [(11/6)*V]/(2V) = 11/12

The correct answer is (C).
Rahul Lakhani
Quant Expert
Gurome, Inc.
https://www.GuroMe.com
On MBA sabbatical (at ISB) for 2011-12 - will stay active as time permits
1-800-566-4043 (USA)
+91-99201 32411 (India)
Top
Newbie | Next Rank: 10 Posts
Posts: 9
Joined: Tue Aug 17, 2010 10:55 am
Thanked: 2 times

by wdgolden » Mon Aug 30, 2010 4:57 am
If I have a drum that is 1/2 full and then double the size of the drum, the drum will now be 1/4 full. So if I poured X which is half full into Y which is twice as big I would end up with Y being 1/4 full. Now just add the 2/3 that's already in Y.

1/4 + 2/3 = 3/12 + 8/12 = 11/12
Top
User avatar
GMAT Instructor
Posts: 1031
Joined: Thu Jul 03, 2008 1:23 pm
Location: Malibu, CA
Thanked: 716 times
Followed by:255 members
GMAT Score:750

by Brian@VeritasPrep » Mon Aug 30, 2010 1:22 pm
Hey nickknock,

Great responses in this thread thus far, but let me tack on one more - if you want to put your mistake into a broader context, you didn't keep track of units, which is a big mistake to make in any word problems.

Had you put units (gallons or liters, for example) on the numbers you plugged in, you would likely have been fine, as then it would have been pretty clear that the number of gallons in one drum had to be half that of the other, and you wouldn't have gone after that common denominator.

When you're working with problems that have units, make sure that you're always aware of what each number represents. The GMAT is much more conceptual than "calculational" - questions are often designed to try to trick you into simply "crunching numbers" when really what the test wants is to determine how well you make decisions. To make good decisions, make sure that you're always aware of what the numbers mean!
Brian Galvin
GMAT Instructor
Chief Academic Officer
Veritas Prep

Looking for GMAT practice questions? Try out the Veritas Prep Question Bank. Learn More.
Top
Post new topic Post Reply
• Page 1 of 1