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Surface of a certain planet

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Surface of a certain planet

by bhumika.k.shah » Fri Jan 22, 2010 2:08 am
The surface of a certain planet reflects 80 percent of the light that strikes it. The clouds
around the planet then absorb 40 percent of the reflected light. What percent of the light
that strikes the planet is reflected from the surface and passes through the clouds without
being absorbed?

A.32%
B.40%
C.48%
D.60%
E.88%

OA C

how ??? me getting E :(
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by thephoenix » Fri Jan 22, 2010 2:26 am
let x be the amnt of light striking

then 0.8x is the amnt of light being reflected

and 0.4*0.8x=0.32 x is the amnt of light being absorbed

and therefore 0.8x-0.32x=0.48x is the amnt of light passing through (or not getting absorbed)

%=0.48x/x=48 ( edited)

IMO C
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by sanju09 » Fri Jan 22, 2010 3:18 am
bhumika.k.shah wrote:The surface of a certain planet reflects 80 percent of the light that strikes it. The clouds
around the planet then absorb 40 percent of the reflected light. What percent of the light
that strikes the planet is reflected from the surface and passes through the clouds without
being absorbed?

A.32%
B.40%
C.48%
D.60%
E.88%

OA C

how ??? me getting E :(
I don't really agree with the OA here.

If 100 is incident, 80 is reflected immediately; and 40 percent of 80 that is 32, gets absorbed in the clouds thereby enabling the remaining 48 out of 80, which is (48/80) × 100 = 60 percent of the light that strikes the planet and is reflected from the surface and passes through the clouds without being absorbed.

Mind what is in bold here. Read again-it clearly asks us about the fraction of light that finally come out of the clouds out of what has been first reflected from the planet from all the incident light.

I'll go with [spoiler]D[/spoiler].
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by bhumika.k.shah » Fri Jan 22, 2010 3:23 am
its a plain wordy problem.thats why it seems soo confusing.
its saying planet absorbs 40% of the reflecting light. which means it doesn't absorb the remaining 60% of the light tht is getting reflected.

let the light strike = 100
therefore 80% of it tht is reflected will be = 80
40% of the light tht is reflected is getting absorbed = 40% of 80 = 32

therefore of the light tht is reflected , 60% is not getting absorbed = 60% of 80 = 48

the question asks what % of the light that is striking is reflected but not getting absorbed = 48/100 = 48%

Hence Answer C :D
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by sanju09 » Fri Jan 22, 2010 3:25 am
bhumika.k.shah wrote:The surface of a certain planet reflects 80 percent of the light that strikes it. The clouds around the planet then absorb 40 percent of the reflected light. What percent of the light that strikes the planet is reflected from the surface and passes through the clouds without being absorbed?
A.32%
B.40%
C.48%
D.60%
E.88%

OA C

how ??? me getting E :(
I don't really agree with the OA here.

If 100 is incident, 80 is reflected immediately; and 40 percent of 80 that is 32, gets absorbed in the clouds thereby enabling the remaining 48 out of 80, which is (48/80) × 100 = 60 percent of the light that strikes the planet and is reflected from the surface and passes through the clouds without being absorbed.

Mind what's in bold above. Read it again-it clearly asks for the fraction of light that has finally come out of clouds without being absorbed, out of what has been first reflected from the planet surface from all the light that was made incident to it.

I'll go with [spoiler]D[/spoiler] here.
The mind is everything. What you think you become. -Lord Buddha



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by sanju09 » Fri Jan 22, 2010 3:28 am
sanju09 wrote:
bhumika.k.shah wrote:The surface of a certain planet reflects 80 percent of the light that strikes it. The clouds
around the planet then absorb 40 percent of the reflected light. What percent of the light
that strikes the planet is reflected from the surface and passes through the clouds without
being absorbed?

A.32%
B.40%
C.48%
D.60%
E.88%

OA C

how ??? me getting E :(
I don't really agree with the OA here.

I'll go with [spoiler]D[/spoiler].
The mind is everything. What you think you become. -Lord Buddha



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by bhumika.k.shah » Fri Jan 22, 2010 3:28 am
kindly read my post above and let me know if u still need some more explanation
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by sanju09 » Fri Jan 22, 2010 3:31 am
NOW I AGREE

SO WHAT?
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by bhumika.k.shah » Fri Jan 22, 2010 3:32 am
hence the answer is C - 48% :-P
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by sanju09 » Fri Jan 22, 2010 3:34 am
bhumika.k.shah wrote:hence the answer is C - 48% :-P
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by blaster » Fri Jan 22, 2010 4:45 am
take for example 100

80% of 100 is 80
then 40% of 80 is 32

the final step 80-32=48
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