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# Water lily grows at such a rate that it covers double the

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Senior Manager
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Water lily grows at such a rate that it covers double the [#permalink]  28 Mar 2004, 16:01
Water lily grows at such a rate that it covers double the surface that it covered the day before. It takes 30 days for a water lily to cover the entire surface of the pond. How long would it take for two water lilies of the same size to cover the entire surface of the same pond?
A. 8.33
B. 12.5
C. 15
D. 20
E. 29
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given x2^30 = 2x2^n, find n

n = 29
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Re: water lily [#permalink]  25 Jul 2006, 06:19
cbrf3 wrote:
Water lily grows at such a rate that it covers double the surface that it covered the day before. It takes 30 days for a water lily to cover the entire surface of the pond. How long would it take for two water lilies of the same size to cover the entire surface of the same pond?
A. 8.33
B. 12.5
C. 15
D. 20
E. 29

Can somebody rephrase the question? I'm a bit confused:(

As I understood the 2 Lilies have to cover the same size pond as 1 Lily had to cover.... in this case woudn't the two lilies cover the same area twice as faster???

in other words, suppose we divide the pond area in two halvs and give each half to each lilly ... they should both cover the whole area twice as faster...

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Hi koSTARica,

My suggestion for you to understand the problem.
Do a bit of reverse engineering. Imagine that today the lily has fully covered the pond. What was the area covered yesterday ( and you will have the answer. )

Hope this helps.

Thanks
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jainvik7 wrote:
Hi koSTARica,

My suggestion for you to understand the problem.
Do a bit of reverse engineering. Imagine that today the lily has fully covered the pond. What was the area covered yesterday ( and you will have the answer. )

Hope this helps.

Thanks

I guess yesterday will be covered 2^29 area....
as i understand lily covers pond in the following way 2*2*.......*2*2 for 30 days.... {2^30}

But I dont get if the second Lily is to help the first Lily cover the area, won't they cover it twice as faster?
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hallelujah1234 wrote:
given x2^30 = 2x2^n, find n

n = 29

one Lily x2^30

two Lilies x2^n+x2^n so it wwill be 2*x2^n => x2^(n+1)

x2^30=x2^(n+1+

30=n+1

n=29

I UNDERSTAND the Mathematics:) but still don't get the idea:((( Maybe too much work today:( Need "F5 Button" (refresh:)
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Let area covered in one day be x

Hence area covered in 30 days is 2^30.x

Area of the pond is 2^30.x
Let two lilies can cover the same area in n days

Hence

2^30.x = 2.2^n.x
n+1 = 30
n = 29
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