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For a light that has an intensity of 60 candles at its source, the int

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For a light that has an intensity of 60 candles at its source, the int [#permalink]

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New post 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 feet from the source is given by the formula \(S=\frac{60k}{d^2}\), 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

OPEN DISCUSSION OF THIS QUESTION IS HERE: https://gmatclub.com/forum/for-a-light- ... 67459.html
[Reveal] Spoiler: OA

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Last edited by Bunuel on 05 Jun 2017, 06:55, edited 1 time in total.
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Re: For a light that has an intensity of 60 candles at its source, the int [#permalink]

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New post 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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Re: For a light that has an intensity of 60 candles at its source, the int [#permalink]

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New post 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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Re: For a light that has an intensity of 60 candles at its source, the int [#permalink]

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New post 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 source, the int [#permalink]

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Re: For a light that has an intensity of 60 candles at its source, the int [#permalink]

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New post 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.

Instead, set 30 = 60k/2^2

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.

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Re: For a light that has an intensity of 60 candles at its source, the int [#permalink]

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New post 05 Jun 2017, 06:42
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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. . ."

mbaapp1234 writes that
Quote:
The wording here is tricky and intended to deceive.

Maybe I'm missing something, but I think it's worse than that. Maybe there's a formatting error?

S=60k/d^2 60

What on earth is the operator, if any, between \(d^2\) and 60? Should 60 be there at all?

What am I missing? Anyone?
bunuel?

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Re: For a light that has an intensity of 60 candles at its source, the int [#permalink]

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New post 05 Jun 2017, 06:55
genxer123 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. . ."

mbaapp1234 writes that
Quote:
The wording here is tricky and intended to deceive.

Maybe I'm missing something, but I think it's worse than that. Maybe there's a formatting error?

S=60k/d^2 60

What on earth is the operator, if any, between \(d^2\) and 60? Should 60 be there at all?

What am I missing? Anyone?
bunuel?

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Re: For a light that has an intensity of 60 candles at its source, the int [#permalink]

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New post 05 Jun 2017, 06:56
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 feet from the source is given by the formula \(S=\frac{60k}{d^2}\), 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


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 feet from the source is given by the formula S=60k/d^2, 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 intensity of the light is 30 candles (S) at a distance of 2 feet (d) from the source: \(30=\frac{60k}{2^2}\) --> \(k=2\).

Therefore the intensity of the light at a distance of 20 feet from the source is \(S=\frac{60*2}{20^2}=\frac{3}{10}\).

Answer: A.

OPEN DISCUSSION OF THIS QUESTION IS HERE: https://gmatclub.com/forum/for-a-light- ... 67459.html
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Collection of Questions:
PS: 1. Tough and Tricky questions; 2. Hard questions; 3. Hard questions part 2; 4. Standard deviation; 5. Tough Problem Solving Questions With Solutions; 6. Probability and Combinations Questions With Solutions; 7 Tough and tricky exponents and roots questions; 8 12 Easy Pieces (or not?); 9 Bakers' Dozen; 10 Algebra set. ,11 Mixed Questions, 12 Fresh Meat

DS: 1. DS tough questions; 2. DS tough questions part 2; 3. DS tough questions part 3; 4. DS Standard deviation; 5. Inequalities; 6. 700+ GMAT Data Sufficiency Questions With Explanations; 7 Tough and tricky exponents and roots questions; 8 The Discreet Charm of the DS; 9 Devil's Dozen!!!; 10 Number Properties set., 11 New DS set.


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Re: For a light that has an intensity of 60 candles at its source, the int   [#permalink] 05 Jun 2017, 06:56
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