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# m01-q37

Author Message
Intern
Joined: 15 May 2012
Posts: 41
Followers: 0

Kudos [?]: 5 [0], given: 94

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17 Dec 2012, 17:15
when 69/36*100 is calculated, we should consider the digits after decimal, or else there are chances of choosing the wrong answer.
Intern
Joined: 17 Sep 2013
Posts: 6
Followers: 0

Kudos [?]: 0 [0], given: 4

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20 Sep 2013, 05:18
pmal04 wrote:
Hi millhouse,
Why is it (new-old)/old ? it should have been new/old. Thanks

the questions asks for "growth" which is the same as "increase" so you only take the % difference.
btw (new-old)/old is the same as [(new/old) - 1]

regards,
Mishari
Verbal Forum Moderator
Joined: 16 Jun 2012
Posts: 1132
Location: United States
Followers: 278

Kudos [?]: 3112 [1] , given: 123

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25 Sep 2013, 01:06
1
KUDOS
pmal04 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?

(A) 140%
(B) 141%
(C) 190%
(D) 192%
(E) 292%

[Reveal] Spoiler: OA
D

Source: GMAT Club Tests - hardest GMAT questions

I don't agree with OA, Can anybody please explain?
When it says 'grow', we need to take the difference between new and old.
However, the OE simply does new/old instead of (new-old)/old.

Before: Area between two circles = A1 - A2 = Pi(10^2 - 8^2) = (10-8)(10+8)Pi = 36Pi
After: Area between two circles = A'1 - A'2 = Pi(11^2 - 4^2) = (11-4)(11+4)Pi = 105Pi

Percentage increase = [105 - 36]/36 * 100%
= 69/36* 100%
= [36 + 33]/36 * 100%
= 100% + 33/36*100%
= 100% + 11/12*100%
= 100% + 92%
= 192%

Tip:
9/10 = 90%;
10/11 = 91%;
11/12 = 92%

This rule helps you pick a correct answer between C and D quickly.

Hope it helps.
_________________

Please +1 KUDO if my post helps. Thank you.

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Re: m01-q37   [#permalink] 25 Sep 2013, 01:06

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# m01-q37

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