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If A and B are each circles, what is the radius of the larger circle?

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If A and B are each circles, what is the radius of the larger circle?  [#permalink]

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New post 09 Sep 2018, 08:04
1
00:00
A
B
C
D
E

Difficulty:

  25% (medium)

Question Stats:

84% (01:29) correct 16% (01:39) wrong based on 40 sessions

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If A and B are each circles, what is the radius of the larger circle?  [#permalink]

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New post 25 Sep 2018, 09:27
1
If A and B are each circles, what is the radius of the larger circle?
Larger Circle radius = R ; Smaller Circle radius = r

1) Larger circle's circumference is 4 times the circumference of the smaller circle.

\(2πR=4(2πr)\)
\(R=4r\)
To find the area we need exact value of r. As R=4 and r can be 1 or R=400 and r =100 we can get different areas for the proportional radii. So not sufficient

2) The area of A minus the area of B is \(20π\)
\(πR^2-πr^2=20π\)
\(R^2-r^2=20\)

From 1 and 2
Equate R=4r into \(R^2-r^2=20\)
Since this is a DS question you can stop here you would be able to find the exact values of r and R so answer is C.



But if its a PS question
\(4r^2-r^2=20\)
\(r^2=4/3\)
\(r=2/√3\)
Then
\(R=8/√3\)
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Re: If A and B are each circles, what is the radius of the larger circle?  [#permalink]

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New post 25 Sep 2018, 09:37
1
NO, The answer can not be E, because statement says "The area of A minus the area of B is 20π"
since the value of the difference is +'ve, that means A is the bigger circle.

AvidDreamer09 wrote:
If A and B are each circles, what is the radius of the larger circle?
Larger Circle radius = R ; Smaller Circle radius = r

1) Larger circle's circumference is 4 times the circumference of the smaller circle.

\(2πR=4(2πr)\)
\(R=4r\)
To find the area we need exact value of r. As R=4 and r can be 1 or R=400 and r =100 we can get different areas for the proportional radii. So not sufficient

2) The area of A minus the area of B is \(20π\)
\(πR^2-πr^2=20π\)
\(R^2-r^2=20\)

From 1 and 2
Equate R=4r into \(R^2-r^2=20\)
Since this is a DS question you can stop here you would be able to find the exact values of r and R so answer is C.

But if its a PS question
\(4r^2-r^2=20\).......=>(4r)^2-r^2=20
\(r^2=4/3\)
\(r=2/√3\)
Then
\(R=8/√3\)
\(πR^2=π8/√3)^2\)

I do however have a doubt
Am i wrong in thinking that Area A can be the bigger circle and area b is the smaller circle. No where in the question is the relation between large small a and b given. So if this question did come up in the test. Could the answer be E????

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Re: If A and B are each circles, what is the radius of the larger circle?   [#permalink] 25 Sep 2018, 09:37
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If A and B are each circles, what is the radius of the larger circle?

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