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...and another one?

by kumard24 » Tue Mar 18, 2008 9:59 am
8. A certain culture of bacteria quadruples every hour. If a container with these bacteria was half full at 10:00 a.m., at what time was it one-eighth full?
(A) 9:00 a.m.
(B) 7:00 a.m.
(C) 6:00 a.m.
(D) 4:00 a.m.
(E) 2:00 a.m.

Is there a link to the 500ps questions which will provide answers
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by xilef » Tue Mar 18, 2008 10:10 am
1/2 is 4 times 1/8, which is exactly the hourly rate of growth, so the answer is A.
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Re: ...and another one?

by Stuart@KaplanGMAT » Tue Mar 18, 2008 1:33 pm
kumard24 wrote:8. A certain culture of bacteria quadruples every hour. If a container with these bacteria was half full at 10:00 a.m., at what time was it one-eighth full?
(A) 9:00 a.m.
(B) 7:00 a.m.
(C) 6:00 a.m.
(D) 4:00 a.m.
(E) 2:00 a.m.

Is there a link to the 500ps questions which will provide answers
Picking numbers is a great way to answer this question.

To make life easy, let's let the full container hold 800 bacteria.

So, 1/2 full would be 400, 1/8 full would be 100.

Working backwards, we divide by 4 each hour.

So, if it held 400 at 10am, it held 400/4 = 100 at 9am. 100 is our target, so 9am is the correct answer to the question.
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