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by gmatmachoman » Mon Feb 21, 2011 7:01 am
A certain culture of bacteria quadruples every hour. If a container with these bacteria was one-eighth full at 10:00 a.m., at what time was it (1/128)th full?


(A) 9:00 a.m.
(B) 7:00 a.m.
(C) 6:00 a.m.
(D) 4:00 a.m.
(E) 8:00 a.m.
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by Night reader » Mon Feb 21, 2011 7:07 am
gmatmachoman wrote:A certain culture of bacteria quadruples every hour. If a container with these bacteria was one-eighth full at 10:00 a.m., at what time was it (1/128)th full?


(A) 9:00 a.m.
(B) 7:00 a.m.
(C) 6:00 a.m.
(D) 4:00 a.m.
(E) 8:00 a.m.
IOM E

was that you posting this :)

x/8 at 10 am;
x/32 at 9 am;
x/128 at 8 am.
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by GMATGuruNY » Mon Feb 21, 2011 7:14 am
gmatmachoman wrote:A certain culture of bacteria quadruples every hour. If a container with these bacteria was one-eighth full at 10:00 a.m., at what time was it (1/128)th full?


(A) 9:00 a.m.
(B) 7:00 a.m.
(C) 6:00 a.m.
(D) 4:00 a.m.
(E) 8:00 a.m.
Plug in container = 128.
Original amount of bacteria = 1/128 * 128 = 1.
Final amount of bacteria = 1/8 * 128 = 16.
After 1 hour, bacteria = 4*1 = 4.
After 2 hours, bacteria = 4*4 = 16.
Since 2 hours are needed, the container was 1/128 full at 10am - 2 = 8am.

The correct answer is E.
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