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If the radius of a circle is decreased by 30 percent, by what percent

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If the radius of a circle is decreased by 30 percent, by what percent [#permalink]

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New post 14 Nov 2017, 22:38
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If the radius of a circle is decreased by 30 percent, by what percent will the area of the circular region be decreased?

(A) 15%
(B) 49%
(C) 51%
(D) 60%
(E) 90%
[Reveal] Spoiler: OA

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Re: If the radius of a circle is decreased by 30 percent, by what percent [#permalink]

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New post 14 Nov 2017, 22:45
let the radius of circle be R
Area of the circle= 3.14 * R^2
Now, radius is decreased by 30%
New radius of circle would be 0.7R
Hence,
New Area= 3.14*(0.7R)^2
New Area= 0.49 * 3.14*R^2

Therefore, New area of the circle would be 51% less than that of original one.
Answer Option C.


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If the radius of a circle is decreased by 30 percent, by what percent [#permalink]

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New post 15 Nov 2017, 03:18
Bunuel wrote:
If the radius of a circle is decreased by 30 percent, by what percent will the area of the circular region be decreased?

(A) 15%
(B) 49%
(C) 51%
(D) 60%
(E) 90%

Choose values

Let the original radius = 10
Original area is \(\pi*r^2=100\pi\)

The radius is decreased by 30 percent:
.70(10) = 7
New area is \(\pi*r^2=49\pi\)

Percent decrease:
\(\frac{New-Old}{Old} * 100\)

\(\frac{100-49}{100}=\frac{51}{100} * 100 = 51\) percent

Answer C

Scale Factor
Length * Length = Area
Scale factor = (1 - .30) = .70

When area (length * length) is decreased by a scale factor, the scale factor, squared (because it is used for both lengths)*, can be used to calculate the percent change.

\((.7)^2 = .49\)
Original = \(1\)
Percent decrease:
\((1 - .49) = .51 * 100 = 51\) percent

Answer C

That is:
Length * length = area = (scale factor)\(^2\) for change
\(\pi\) is constant
Radius is the length affected twice by scale factor.
Original, \(r^2=(r)(r)\) to new, \((.7r)^2=(.7r)(.7r)\)

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Re: If the radius of a circle is decreased by 30 percent, by what percent [#permalink]

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New post 15 Nov 2017, 07:38
Bunuel wrote:
If the radius of a circle is decreased by 30 percent, by what percent will the area of the circular region be decreased?

(A) 15%
(B) 49%
(C) 51%
(D) 60%
(E) 90%

\(-30 - 30 + \frac{(-30*-30)}{100} = 51\)%

Answer will be (C)

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Re: If the radius of a circle is decreased by 30 percent, by what percent   [#permalink] 15 Nov 2017, 07:38
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