An antibiotic is added to a culture of bacteria and is found to kill 1/3 of the bacteria in the culture every 30 seconds. If a lab assistant checks the culture after 2 minutes, what fraction of the original bacteria will have been killed?
a) 16/81
b) 8/27
c) 19/27
d) 65/81
e) 80/81
Antibiotic
This topic has expert replies
-
- Master | Next Rank: 500 Posts
- Posts: 161
- Joined: Mon Apr 05, 2010 9:06 am
- Location: Mumbai
- Thanked: 37 times
Hi,
At start, # of bacteria = x
After 30 seconds, # of bacteria left = (2/3) * x
After 60 seconds, # of bacteria left = (2/3) * (2/3) * x
After 90 seconds, # of bacteria left = (2/3) * (2/3) * (2/3) * x
After 120 seconds, # of bacteria left = (2/3) * (2/3) * (2/3) * (2/3) * x = (16/81) x
Those killed = x - (16/81)x = (65/81) * x
Is it D ? OA please ... Thanks.
At start, # of bacteria = x
After 30 seconds, # of bacteria left = (2/3) * x
After 60 seconds, # of bacteria left = (2/3) * (2/3) * x
After 90 seconds, # of bacteria left = (2/3) * (2/3) * (2/3) * x
After 120 seconds, # of bacteria left = (2/3) * (2/3) * (2/3) * (2/3) * x = (16/81) x
Those killed = x - (16/81)x = (65/81) * x
Is it D ? OA please ... Thanks.
Naveenan Ramachandran
4GMAT, Dadar(W) & Ghatkopar(W), Mumbai
4GMAT, Dadar(W) & Ghatkopar(W), Mumbai
- GMATGuruNY
- 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
We can plug in a value for the bacteria.N:Dure wrote:An antibiotic is added to a culture of bacteria and is found to kill 1/3 of the bacteria in the culture every 30 seconds. If a lab assistant checks the culture after 2 minutes, what fraction of the original bacteria will have been killed?
a) 16/81
b) 8/27
c) 19/27
d) 65/81
e) 80/81
Let bacteria = 81.
After first 30 seconds, 1/3*81 = 27 killed, 81-27 = 54 left.
After next 30 seconds, 1/3*54 = 18 killed, 54-18 = 36 left.
After next 30 seconds, 1/3*36 = 12 killed, 36-12 = 24 left.
After next 30 seconds, 1/3*24 = 8 killed, 24-8 = 16 left.
Since only 16 are left, 81-16 = 65 were killed.
Killed/Total = 65/81.
The correct answer is D.
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
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