Check GMAT Club Decision Tracker for the Latest School Decision Releases https://gmatclub.com/AppTrack

 It is currently 24 May 2017, 06:40

### GMAT Club Daily Prep

#### Thank you for using the timer - this advanced tool can estimate your performance and suggest more practice questions. We have subscribed you to Daily Prep Questions via email.

Customized
for You

we will pick new questions that match your level based on your Timer History

Track

every week, we’ll send you an estimated GMAT score based on your performance

Practice
Pays

we will pick new questions that match your level based on your Timer History

# Events & Promotions

###### Events & Promotions in June
Open Detailed Calendar

# All scientists expect the population of penguins on the

 post reply Question banks Downloads My Bookmarks Reviews Important topics
Author Message
GMAT Instructor
Joined: 04 Jul 2006
Posts: 1263
Followers: 29

Kudos [?]: 307 [0], given: 0

All scientists expect the population of penguins on the [#permalink]

### Show Tags

09 Jul 2006, 02:30
00:00

Difficulty:

(N/A)

Question Stats:

0% (00:00) correct 0% (00:00) wrong based on 0 sessions

### HideShow timer Statistics

This topic is locked. If you want to discuss this question please re-post it in the respective forum.

All scientists expect the population of penguins on the island of Zeeberg is expected to rise by from 50,000 today to 800,000 18 years from now. Scientist A holds that the population will follow an arithmetic progression, that is that the absolute increase in population from year to year will be constant. Scientist B says that it will follow a geometric progression, which is to say that the percentage increase in population from year to year will be constant. If the two scientists are asked to predict the population of penguins in 9 yearsâ€™ time, by how much will their predictions differ?

(A) 150,000 (B) 225,000 (C) 300,000 (D) 425,000 (E) none of these
Manager
Joined: 12 May 2006
Posts: 183
Followers: 1

Kudos [?]: 64 [0], given: 0

### Show Tags

09 Jul 2006, 10:45
For AP

Tn = a + (n-1)*d
T18 = 800,000 = 50,000 + (18-1) * d
=> d = 750,000/17

For GP

Tn = a * r ^ (n-1)
T18 = 800,000 = 50,000 * r ^ (18-1)
=> r^17 = 16 => r = 16 ^ 1/17

Now calculating the 9th term for the AP and GP

For AP

T9 = a*r^8 = 50,000 * 16 ^ (8/17) ~ 50,000 * 16^(1/2) = 200,000

For GP

T9 = a + 8d = 50,000+8*(750,000/17)~ 50,000 + (750,000/2) = 420,000

The difference between to T9s

= 420,000 - 200,000 = 220,000

Hence the closest answer is 225,000.
Manager
Joined: 08 Jul 2006
Posts: 89
Followers: 1

Kudos [?]: 0 [0], given: 0

### Show Tags

09 Jul 2006, 15:28
grepro wrote:

Now calculating the 9th term for the AP and GP

For GP

T9 = a*r^8 = 50,000 * 16 ^ (8/17) ~ 50,000 * 16^(1/2) = 200,000

For AP

T9 = a + 8d = 50,000+8*(750,000/17)~ 50,000 + (750,000/2) = 420,000

The difference between to T9s

= 420,000 - 200,000 = 220,000

Hence the closest answer is 225,000.

I follow your methodology until calculating for GP
How does 16 ^ (8/17) become 16 ^ (1/2)
It must be my weakness with exponent properties coming through here...

GMAT Instructor
Joined: 04 Jul 2006
Posts: 1263
Followers: 29

Kudos [?]: 307 [0], given: 0

### Show Tags

09 Jul 2006, 15:41
grepro wrote:
For AP

Tn = a + (n-1)*d
T18 = 800,000 = 50,000 + (18-1) * d
=> d = 750,000/17

For GP

Tn = a * r ^ (n-1)
T18 = 800,000 = 50,000 * r ^ (18-1)
=> r^17 = 16 => r = 16 ^ 1/17

Now calculating the 9th term for the AP and GP

For AP

T9 = a*r^8 = 50,000 * 16 ^ (8/17) ~ 50,000 * 16^(1/2) = 200,000

For GP

T9 = a + 8d = 50,000+8*(750,000/17)~ 50,000 + (750,000/2) = 420,000

The difference between to T9s

= 420,000 - 200,000 = 220,000

Hence the closest answer is 225,000.

I assume you are saying that T1 is the value now, at the beginning of the first year. Then T19=800,000

so r^18=16=4^2 and r^9=4
d=750,000/18 and 9d=375,000
Manager
Joined: 26 Jun 2006
Posts: 152
Followers: 1

Kudos [?]: 7 [0], given: 0

### Show Tags

09 Jul 2006, 18:14
I thought that Geometric Progression is out of scope of GMAt. Am I wrong? Should I put it back on my "cheat-sheet"?
CEO
Joined: 20 Nov 2005
Posts: 2899
Schools: Completed at SAID BUSINESS SCHOOL, OXFORD - Class of 2008
Followers: 25

Kudos [?]: 288 [0], given: 0

### Show Tags

09 Jul 2006, 20:49
The total terms are 19 not 18.

Using this I got 10th term (i.e after 9 years) of AP = 375,000

The 10th term for GP is = 200,000

Difference = 275,000

So answer should be E.
_________________

SAID BUSINESS SCHOOL, OXFORD - MBA CLASS OF 2008

09 Jul 2006, 20:49
Display posts from previous: Sort by

# All scientists expect the population of penguins on the

 post reply Question banks Downloads My Bookmarks Reviews Important topics

 Powered by phpBB © phpBB Group and phpBB SEO Kindly note that the GMAT® test is a registered trademark of the Graduate Management Admission Council®, and this site has neither been reviewed nor endorsed by GMAC®.