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

Redeem

Gorillas in a zoo! Math Problem

This topic has expert replies
Post new topic Post Reply
Previous Topic Next Topic
User avatar
Newbie | Next Rank: 10 Posts
Posts: 1
Joined: Mon Oct 31, 2011 8:55 pm

Gorillas in a zoo! Math Problem

by IMARTIN9 » Wed Aug 10, 2016 2:08 pm
How would you solve this using the 'ratio box' study trick?



The ratio of gorillas to lions in a zoo was 5:4. After transferring 13 of the lions to another zoo, the new ratio of gorillas to lions is 7:3. How many gorillas currently reside in the zoo?


I tried setting it up like this, picking at random the total # of animals to be 45

Gorillas Lions Total
5 4 9

5 5

25 20 45



however, when I use those #s, the 7:# ratio box does not work out?

Thank you!!!!
Top
Source: — Problem Solving |
User avatar
Legendary Member
Posts: 2663
Joined: Wed Jan 14, 2015 8:25 am
Location: Boston, MA
Thanked: 1153 times
Followed by:128 members
GMAT Score:770

by DavidG@VeritasPrep » Wed Aug 10, 2016 3:08 pm
IMARTIN9 wrote:How would you solve this using the 'ratio box' study trick?



The ratio of gorillas to lions in a zoo was 5:4. After transferring 13 of the lions to another zoo, the new ratio of gorillas to lions is 7:3. How many gorillas currently reside in the zoo?


I tried setting it up like this, picking at random the total # of animals to be 45

Gorillas Lions Total
5 4 9

5 5

25 20 45



however, when I use those #s, the 7:# ratio box does not work out?

Thank you!!!!
Initially:
Gorillas: 5x
Lions: 4x

After 13 lions shipped out:
Gorillas: 5x
Lions: 4x - 13

If the ratio of Gorillas to Lions is now 7:3, then 5x/(4x-13) = 7/3
Cross-multiply: 15x = 28x - 91
-13x = -91
x = 7

Gorillas: 5x = 5*7 = 35
Veritas Prep | GMAT Instructor

Veritas Prep Reviews
Save $100 off any live Veritas Prep GMAT Course
Top
User avatar
Legendary Member
Posts: 2663
Joined: Wed Jan 14, 2015 8:25 am
Location: Boston, MA
Thanked: 1153 times
Followed by:128 members
GMAT Score:770

by DavidG@VeritasPrep » Wed Aug 10, 2016 3:12 pm
IMARTIN9 wrote:How would you solve this using the 'ratio box' study trick?



The ratio of gorillas to lions in a zoo was 5:4. After transferring 13 of the lions to another zoo, the new ratio of gorillas to lions is 7:3. How many gorillas currently reside in the zoo?


I tried setting it up like this, picking at random the total # of animals to be 45

Gorillas Lions Total
5 4 9

5 5

25 20 45



however, when I use those #s, the 7:# ratio box does not work out?

Thank you!!!!
Note also that if you began with 25 gorillas and 20 lions and removed 13 lions, you'd have 25 gorillas and 7 lions, which is clearly not a 7:3 ratio. Moreover, the initial ratio tells us that the number of gorillas is a multiple of 5 and the new ratio tells us that the number of gorillas is a multiple of 7, so the smallest possible value for the number of gorillas is 35. If 5x = 35, x = 7. So the initial number of lions would be 4x = 4*7 = 28. If you removed 13, you'd have 15 lions. 35/15 = 7/3.
Veritas Prep | GMAT Instructor

Veritas Prep Reviews
Save $100 off any live Veritas Prep GMAT Course
Top

GMAT/MBA Expert

User avatar
Elite Legendary Member
Posts: 10392
Joined: Sun Jun 23, 2013 6:38 pm
Location: Palo Alto, CA
Thanked: 2867 times
Followed by:511 members
GMAT Score:800

by [email protected] » Wed Aug 10, 2016 4:14 pm
Hi IMARTIN9,

What is the source of this question? If it's a GMAT question, then it should include 5 answer choices. In many cases, the answer choices provide a clue as to how you might go about solving the prompt. Since this question asks for the number of gorillas that currently reside in the zoo, we could use those answers 'against' the prompt and find the one that fits all of the given information. Without having those answers though, we're forced to approach this prompt algebraically (which is sometimes not the fastest way to get to the correct answer).

GMAT assassins aren't born, they're made,
Rich
Contact Rich at [email protected]
Image
Top
User avatar
GMAT Instructor
Posts: 15539
Joined: Tue May 25, 2010 12:04 pm
Location: New York, NY
Thanked: 13060 times
Followed by:1906 members
GMAT Score:790

by GMATGuruNY » Sat Aug 13, 2016 8:06 pm
The ratio of gorillas to lions in a zoo was 5:4. After transferring 13 of the lions to another zoo, the new ratio of gorillas to lions is 7:3. How many gorillas currently reside in the zoo?
a) 35
b) 50
c) 70
d) 90
e) 105
We can PLUG IN THE ANSWERS, which represent the number of gorillas currently in the zoo.

Since G:L = 7:3, the number of gorillas must be a multiple of 7.
Eliminate B and D.

3 options remain:
A --> G=35, L=15 (since 7:3 = 35:15)
C --> G=70, L=30 (since 7:3 = 70:30)
E --> G=105, L=45 (since 7:3 = 105:45)

Before the transfer, G:L = 5:4.
Thus, the number of lions before the transfer must be a multiple of 4.
Before the transfer, there were 13 more lions:
A --> original L = 15+13 = 28.
C --> original L = 30+13 = 43.
E --> original L = 45+13 = 58.
Only A yields a multiple of 4 for the original number of lions.

The correct answer is A.
Private tutor exclusively for the GMAT and GRE, with over 20 years of experience.
Followed here and elsewhere by over 1900 test-takers.
I have worked with students based in the US, Australia, Taiwan, China, Tajikistan, Kuwait, Saudi Arabia -- a long list of countries.
My students have been admitted to HBS, CBS, Tuck, Yale, Stern, Fuqua -- a long list of top programs.

As a tutor, I don't simply teach you how I would approach problems.
I unlock the best way for YOU to solve problems.

For more information, please email me (Mitch Hunt) at [email protected].
Student Review #1
Student Review #2
Student Review #3
Top
GMAT Instructor
Posts: 2630
Joined: Wed Sep 12, 2012 3:32 pm
Location: East Bay all the way
Thanked: 625 times
Followed by:119 members
GMAT Score:780

by Matt@VeritasPrep » Fri Aug 19, 2016 1:44 am
I'd go with

5x / (4x - 13) = 7 / 3

then cross multiply

3*5x = 7*(4x - 13)

15x = 28x - 91

91 = 13x

7 = x

Since we started with 5x gorillas, we've got 5*7, or 35.

Pretty quick and painless, and keeps the problem in one variable.
Top
Post new topic Post Reply
• Page 1 of 1