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Circle B’s diameter was multiplied by 1.8. By what percent, approximat

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Circle B’s diameter was multiplied by 1.8. By what percent, approximat [#permalink]

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New post 18 Sep 2017, 13:07
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Circle B’s diameter was multiplied by 1.8. By what percent, approximately, was the area increased?
A 80%
B 125%
C 225%
D 325%
E 375%
[Reveal] Spoiler: OA

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Re: Circle B’s diameter was multiplied by 1.8. By what percent, approximat [#permalink]

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New post 18 Sep 2017, 14:26
Correct Answer :- 225 % , Area = pi*r^2 , and as diameter is multiplied by 1.8 , Final Area would be 1.8^2 time Initial Area , i.e 3.24 A i Total increase in area :- 3.24 - 1 = 2.24 A , and thus % increase :- 225 %


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Circle B’s diameter was multiplied by 1.8. By what percent, approximat [#permalink]

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New post 18 Sep 2017, 15:35
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SajjadAhmad wrote:
Circle B’s diameter was multiplied by 1.8. By what percent, approximately, was the area increased?
A 80%
B 125%
C 225%
D 325%
E 375%

Use \(k^2\)

Area, including for circles, equals length * length.

If given a length and a scale factor \(k\), to find the percent increase in area from an increase in length, use \(k^2\) (what happens to one length also happens to the other length).

Here: (1.8)(1.8) = 3.24

This scale factor includes the original 1. Percent increase is

\(\frac{New - Old}{Old}\) * 100 =

\(\frac{3.24 - 1}{1}\) * 100 = 224%

ANSWER C

Choose values

You could test numbers and use fractions.

The diameter is multiplied by 1.8 = \(\frac{180}{100}\) = \(\frac{18}{10}\)

So let D = 10

Original area? r = 5, area is \(25\pi\)

New area? New D = 18, r = 9, area is \(81\pi\)

Percent increase: \(\frac{81-25}{25}\) = \(\frac{56}{25}\) * 100 =

224 percent

ANSWER C

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Circle B’s diameter was multiplied by 1.8. By what percent, approximat   [#permalink] 18 Sep 2017, 15:35
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