the lake is full of flowers

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sanju09: GMAT Instructor; Posts: 3650; Joined: Wed Jan 21, 2009 4:27 amFollowed by: 80 members; Location: India; Thanked: 267 times; GMAT Score:760

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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outreach: Legendary Member; Posts: 748; Joined: Sun Jan 31, 2010 7:54 amFollowed by: 3 members; Thanked: 46 times

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

B

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Brent@GMATPrepNow: GMAT Instructor; Posts: 16206; Joined: Mon Dec 08, 2008 6:26 pmFollowed by: 1268 members; Location: Vancouver, BC; Thanked: 5254 times; GMAT Score:770

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

[spoiler]Source: winningprep.com[/spoiler]

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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