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# For a light that has an intensity of 60 candles at its sourc

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For a light that has an intensity of 60 candles at its sourc [#permalink]

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20 Jan 2006, 16:25
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For a light that has an intensity of 60 candles at its source, the intensity in candles, S, of the light at a point d d feet from the source is given by the formula S=60k/d^2 60, where k is a constant. If the intensity of the light is 30 candles at a distance of 2 feet from the source, what is the intensity of the light at a distance of 20 feet from the source?

A. 3/10 candles
B. 1/2 candles
C. 1 1/3 candles
D. 2 candles
E. 3 candles
[Reveal] Spoiler: OA

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20 Jan 2006, 17:43
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ANS = 3/10

S= 60k/d^2

At 2 feet
30 = 60k/4 so k = 2

At 20 feet
S = 60 *2/20^2 = 3/10
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27 Jan 2013, 07:53
ps_dahiya wrote:
At 2 feet
30 = 60k/4 so k = 2

Why 30 equals to that? I can't seem to get it.. can you explain in more detail, pls?
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27 Jan 2013, 12:49
sianissimo wrote:
ps_dahiya wrote:
At 2 feet
30 = 60k/4 so k = 2

Why 30 equals to that? I can't seem to get it.. can you explain in more detail, pls?

S=60K/d^2

Now... 30 when distance 2... 30 = 60k/4 or 60K = 30 *4
Now...Distance 20 then....S = 60k/20^2 = 30*4/(20*20) = 3/10
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Re: For a light that has an intensity of 60 candles at its sourc [#permalink]

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03 Oct 2015, 07:27
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Re: For a light that has an intensity of 60 candles at its sourc [#permalink]

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25 May 2017, 19:46
rlevochkin wrote:
For a light that has an intensity of 60 candles at its source, the intensity in candles, S, of the light at a point d d feet from the source is given by the formula S=60k/d^2 60, where k is a constant. If the intensity of the light is 30 candles at a distance of 2 feet from the source, what is the intensity of the light at a distance of 20 feet from the source?

A. 3/10 candles
B. 1/2 candles
C. 1 1/3 candles
D. 2 candles
E. 3 candles

The wording here is tricky and intended to deceive. If you notice, the light has an intensity of 60 candles at its source but S is not equal to 60; it's simply equal to the intensity. So don't set 60 = 60k/d^2 to solve for the constant.

120 = 60 k
k = 2

Now we can solve for the intensity of this light at a distance of 20 feet: 60(2)/20^2 = 120 /20*20 = 12/40 = 3/10.
Re: For a light that has an intensity of 60 candles at its sourc   [#permalink] 25 May 2017, 19:46
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