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There are two concentric circles with radii 10 and 8. If the radius of

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Intern
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Joined: 09 Feb 2010
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There are two concentric circles with radii 10 and 8. If the radius of  [#permalink]

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New post 31 Aug 2010, 13:21
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There are two concentric circles with radii 10 and 8. If the radius of the outer circle is increased by 10% and the radius of the inner circle decreased by 50%, by what percent does the area between the circles grow?

140%
141%
190%
192%
292%

If i calculation 105/36 i get 2.91 and that * 100 gives 291 or approx 292%

but the correct answer is not that
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Re: There are two concentric circles with radii 10 and 8. If the radius of  [#permalink]

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New post 31 Aug 2010, 14:03
zest4mba wrote:
There are two concentric circles with radii 10 and 8. If the radius of the outer circle is increased by 10% and the radius of the inner circle decreased by 50%, by what percent does the area between the circles grow?



140%
141%
190%
192%
292%

If i calculation 105/36 i get 2.91 and that * 100 gives 291 or approx 292%

but the correct answer is not that


Your calculations are fine, but you then need to answer the question asked. The question is about percent increase. You've proven that the new area is 2.92 times the old area, but that is only an increase of 192%.
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Re: There are two concentric circles with radii 10 and 8. If the radius of  [#permalink]

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New post 01 Sep 2010, 00:45
Remember: % increase/decrease=change/starting point
Intern
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Re: There are two concentric circles with radii 10 and 8. If the radius of  [#permalink]

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New post 21 Apr 2019, 17:01
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Re: There are two concentric circles with radii 10 and 8. If the radius of  [#permalink]

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New post 21 Apr 2019, 19:07
1
zest4mba wrote:
There are two concentric circles with radii 10 and 8. If the radius of the outer circle is increased by 10% and the radius of the inner circle decreased by 50%, by what percent does the area between the circles grow?

140%
141%
190%
192%
292%

If i calculation 105/36 i get 2.91 and that * 100 gives 291 or approx 292%

but the correct answer is not that


Original area between the two circles=\(\pi*(10^2-8^2)=\pi*6^2\)
Area between the two circles upon change in radii=\(\pi*(11^2-4^2)=\pi*105\)

%change in Area=(Change in Area/Original Area)*100=\(\frac{(\pi*105-\pi*36)}{\pi*36}*100\)=191.66% or 192%(Approx.)

Ans. (D)
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Re: There are two concentric circles with radii 10 and 8. If the radius of   [#permalink] 21 Apr 2019, 19:07
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There are two concentric circles with radii 10 and 8. If the radius of

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