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luminous flux

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by GmatKiss » Sun Aug 21, 2011 12:24 am
The "luminous flux," or perceived brightness, of a light source is measured in
lumens and is inversely proportional to the square of the distance from the light. If
a light source produces 200 lumens at a distance of 3 meters, at what distance will
the light source produce a luminous flux of 25 lumens?

3R2
9R3
6R2
6R3
8R2

Please read 3R2 as 3(ROOT(2))
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by Anurag@Gurome » Sun Aug 21, 2011 1:28 am
GmatKiss wrote:The "luminous flux," or perceived brightness, of a light source is measured in lumens and is inversely proportional to the square of the distance from the light. If a light source produces 200 lumens at a distance of 3 meters, at what distance will the light source produce a luminous flux of 25 lumens?
As the luminous flux is inversely proportional to the square of the distance from the light, then the ratio of the luminous flux of a light at two different distances will be equal to the inverse ratio of the square of the distances.

Hence, 200/25 = d²/3², where d is the required distance where the flux will be 25 lumens.

Hence, d² = (3²)*(200/25) = 8*3²
-----> d = √(8*3²) = 6√2

The correct answer is C.
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