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the lake is full of flowers

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the lake is full of flowers

by sanju09 » Sat Jul 31, 2010 5:59 am
A lake has a rare breed of lotuses that triple in number every minute. If one third of the lake is full of flowers in 30 minutes, what is the total time taken for the whole lake to be full?
A. 1 minute
B. 31 minutes
C. 90 minutes
D. 60 minutes
E. 30 minutes

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by outreach » Sat Jul 31, 2010 8:06 am
if x is no of flowers in 1 minute
3x - 2 min
9x - 3 min
pattern
3^(n-1)*x=n min

(3^29)*x=30 min => 1/3 lake
in next minute it will fill the full lake

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by Brent@GMATPrepNow » Sat Jul 31, 2010 8:09 am
sanju09 wrote:A lake has a rare breed of lotuses that triple in number every minute. If one third of the lake is full of flowers in 30 minutes, what is the total time taken for the whole lake to be full?
A. 1 minute
B. 31 minutes
C. 90 minutes
D. 60 minutes
E. 30 minutes

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This is a version of a popular question.
After 30 minutes, the population fills up 1/3 of the lake.
If the population triples every minute, then in 1 more minute, the population will be 3 times 1/3, which equals 1.
So, it will take 1 more minute for the lake to be full.
The total time is 30+1 = 31 (B)
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