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

Redeem

Target Test Prep · GMAT

Choose how you want to prepare

Learn live with an expert or move at your own pace. Every option includes the complete TTP study system.

★★★★★5.0559 reviews
GMATLiveTeach 7 seats left
Chris Peckover
NEXT LIVE COHORT

Oct 13 to Jan 7, 2027

with Chris Peckover

Schedule
Tue, Thu · 8:00 to 10:00 PM ET
Included
40 live hours + 6 months of GMAT OnDemand
  • Live instruction and real-time questions
  • Class recordings and assigned practice
View class & enroll
Limited cohort · enrollment openTarget Test Prep
EALiveTeach 5 seats left
Logan Thompson
EXECUTIVE ASSESSMENT

Sep 6 to Dec 6, 2026

with Logan Thompson

Schedule
Sun · 9:30 AM to 12:30 PM ET
Included
40 hours of live online classes plus six months of access to the complete TTP EA OnDemand course.
  • 165+ EA Score Guarantee
  • 4,100+ Quant, Verbal, and Integrated Reasoning practice questions
  • 400+ hours of in-depth video lessons
  • 3,000+ step-by-step video solutions
View EA class & enroll
Limited cohort · enrollment openTarget Test Prep
GMATOnDemand Start anytime
SELF-PACED MASTERCLASS

Target Test Prep GMAT OnDemand

Complete access from day one. Study on your schedule.

715+ score guarantee
$0to start then $127/mo
  • Personalized study plan and analytics
  • Thousands of lessons and practice questions
Start free 5-day trial
Read 559 reviewsTarget Test Prep

Compare the format, schedule, and included access before enrolling. Prices and seat counts shown reflect the supplied offer details.

Seeking alternative answer method to OG Question

Expert replies
Post new topic Post Reply
Previous Topic Next Topic
Junior | Next Rank: 30 Posts
Posts: 16
Joined: Fri Feb 21, 2014 9:38 am
by crisprin117 » Mon Feb 24, 2014 7:52 am
OG Quant #90 Page 113 Problem Solving

The answer explanation provided is to set up a table and calculate each iteration of bacteria until it reaches 500,000. Can someone please provide me with a mathematical explanation. THANKS!

The population of a bacteria culture doubles every
2 minutes. Approximately how many minutes will
it take for the population to grow from 1,000 to
500,000 bacteria?
(A) 10
(8) 12
(C) 14
(D) 16
(E) 18
Top
Join the discussion
Log in to postLogin with Googleuse password
QUOTE IN REPLY
Source: — Problem Solving |
User avatar
Legendary Member
Posts: 1556
Joined: Tue Aug 14, 2012 11:18 pmFollowed by: 34 members
Thanked: 448 times
GMAT Score:650

by theCodeToGMAT » Mon Feb 24, 2014 8:49 am
1000 (2^0 + 2^1 + 2^2 + 2^3 + 2^4 + 2^5 + ... ) = 500000

2^0 + 2^1 + 2^2 + 2^3 + 2^4 + 2^5 + .. = 500

(1)((2)^n - 1)/(2-1) = 500 [GP Formula]

2^n - 1 = 500
2^n = 501
n is nearly 9
So, total 9*2 = 18 minutes

[spoiler]{E}[/spoiler]?
R A H U L
Top
Join the discussion
Log in to postLogin with Googleuse password
QUOTE IN REPLY
User avatar
Master | Next Rank: 500 Posts
Posts: 359
Joined: Wed Mar 11, 2009 4:37 amFollowed by: 2 members
Location: Kolkata, India
Thanked: 50 times

by Abhishek009 » Mon Feb 24, 2014 8:55 am
crisprin117 wrote:OG Quant #90 Page 113 Problem Solving

The answer explanation provided is to set up a table and calculate each iteration of bacteria until it reaches 500,000. Can someone please provide me with a mathematical explanation. THANKS!

The population of a bacteria culture doubles every
2 minutes. Approximately how many minutes will
it take for the population to grow from 1,000 to
500,000 bacteria?
(A) 10
(8) 12
(C) 14
(D) 16
(E) 18
2 ---> 4 ---> 8 ---> 16 .....

Sn = a + ar +ar^2 + ar^3 ...+ ar^(n-1)

Sn = a((r^n) - 1)/(r-1)

500000 = 1000((2^n) - 1)

=> 500 = 2^n - 1

=> 501 = 2^n

=> n ~ 9


Hence time required will be n *2 = > 18 minutes...
Abhishek
Top
Join the discussion
Log in to postLogin with Googleuse password
QUOTE IN REPLY
User avatar
GMAT Instructor
Posts: 15539
Joined: Tue May 25, 2010 12:04 pmFollowed by: 1906 members
Location: New York, NY
Thanked: 13060 times
GMAT Score:790

by GMATGuruNY » Mon Feb 24, 2014 11:10 am
crisprin117 wrote:OG Quant #90 Page 113 Problem Solving

The answer explanation provided is to set up a table and calculate each iteration of bacteria until it reaches 500,000. Can someone please provide me with a mathematical explanation. THANKS!

The population of a bacteria culture doubles every
2 minutes. Approximately how many minutes will
it take for the population to grow from 1,000 to
500,000 bacteria?
(A) 10
(8) 12
(C) 14
(D) 16
(E) 18
We could use the formula for exponential growth:

Final Amount = Original Amount * (Multiplier)^(Number of changes)

In the problem above:
Final Amount = 500,000
Original Amount = 1000
Multiplier = 2 (since the population keeps doubling)
Number of changes = x.

Plugging these values into the formula, we get:
500,000 = 1000 * 2^x
2^x = 500
Since 2� = 512, x ≈ 9.

Since the number of changes = 9, and each change takes 2 minutes, the total number of minutes = 9*2 = 18.

The correct answer is E.

Similar problems:

https://www.beatthegmat.com/ps-exponent-t115416.html
https://www.beatthegmat.com/word-problem ... 14634.html
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
Join the discussion
Log in to postLogin with Googleuse password
QUOTE IN REPLY

GMAT/MBA Expert

User avatar
GMAT Instructor
Posts: 16207
Joined: Mon Dec 08, 2008 6:26 pmFollowed by: 1268 members
Location: Vancouver, BC
Thanked: 5254 times
GMAT Score:770

by Brent@GMATPrepNow » Mon Feb 24, 2014 12:03 pm
crisprin117 wrote: The answer explanation provided is to set up a table and calculate each iteration of bacteria until it reaches 500,000. Can someone please provide me with a mathematical explanation. THANKS!

The population of a bacteria culture doubles every
2 minutes. Approximately how many minutes will
it take for the population to grow from 1,000 to
500,000 bacteria?
(A) 10
(8) 12
(C) 14
(D) 16
(E) 18
If you want the SUPER MATHEMATICAL solution (i.e., answer the question to several decimal places), you'll need to use logarithms to solve the equation 2^x = 500 [when we do so, we get: xlog2 = log500, which we can rewrite as x = log500/log2. When we evaluate, we get x = 8.97, so the number of minutes = 2(8.97)]


Of course, logs are NOT tested on the GMAT, so this point is somewhat moot :-)

Cheers,
Brent
Brent Hanneson - Creator of GMATPrepNow.com
Image
Top
Join the discussion
Log in to postLogin with Googleuse password
QUOTE IN REPLY
User avatar
GMAT Instructor
Posts: 1248
Joined: Thu Mar 29, 2012 2:57 pmFollowed by: 192 members
Location: Everywhere
Thanked: 503 times
GMAT Score:780

by Bill@VeritasPrep » Mon Feb 24, 2014 12:57 pm
Personally, I think I'd stick with the table method.
Join Veritas Prep's 2010 Instructor of the Year, Matt Douglas for GMATT Mondays

Visit the Veritas Prep Blog

Try the FREE Veritas Prep Practice Test
Top
Join the discussion
Log in to postLogin with Googleuse password
QUOTE IN REPLY

GMAT/MBA Expert

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

by [email protected] » Mon Feb 24, 2014 5:52 pm
Hi crisprin117,

While it's understandable why you'd be interested in knowing the "math" behind a particular question, GMAT questions are written in such a way so that there is usually more than one approach to solving a given Quant (or Verbal) question. It's also worth noting that the Quant section of the GMAT is NOT a math Test; as such it takes more than just "math skills" to hit the highest scores in that section of the Test.

Your goal with any given question should be:
1) Get the question right, if possible
2) Do so in the fastest way possible

With this question, you can essentially "count doubles" and find the answer. This method is probably faster than any other approach that you might take. You can even speed up by ignoring the last 3 zeroes.

1k
becomes
2k
4k
8k
16k
32k
64k
128k
256k
512k

9 "doubles" at 2 minutes each = about 18 minutes

Final Answer: E

GMAT assassins aren't born, they're made,
Rich
Contact Rich at [email protected]
Image
Top
Join the discussion
Log in to postLogin with Googleuse password
QUOTE IN REPLY
Junior | Next Rank: 30 Posts
Posts: 16
Joined: Fri Feb 21, 2014 9:38 am

by crisprin117 » Thu Mar 06, 2014 2:17 pm
Thanks everyone,all the feedback provides me with some perspective. I need to work on efficiency rather than accuracy.

I'm also rereading some sections of the Manhattan Gmat text and have an issue with a particular example in the Word Problems Guide.

Page 126:

Five identical pieces of wire are soldered together to form a longer wire, with the
pieces overlapping by 4 cm at each join. If the wire thus made is exactly 1 meter
long, how long is each of the identical pieces? (1 meter = 100 cm)

MG then states the total length of all the wires is 100 + 4(4) = 116 cm. I don't think takes into account that the wires are layered such that at each 4 cm layer occurs on two wires. I think it should be 100 + 4(4*2) = 132. Can anyone shed some light on this?
Top
Join the discussion
Log in to postLogin with Googleuse password
QUOTE IN REPLY
GMAT Instructor
Posts: 2630
Joined: Wed Sep 12, 2012 3:32 pmFollowed by: 119 members
Location: East Bay all the way
Thanked: 625 times
GMAT Score:780

by Matt@VeritasPrep » Thu Mar 06, 2014 5:19 pm
Crisprin, I tried it visually, and made x the length of each wire. Does this make sense? (Pic attached.)

Image
Top
Join the discussion
Log in to postLogin with Googleuse password
QUOTE IN REPLY
Post new topic Post Reply
• Page 1 of 1