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Percentages problem

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by Mogorosi » Tue Nov 30, 2010 5:57 am
Concrete mixture A is composed of 35 percent cement, 40 percent sand, and 25 percent gravel; concrete mixture B is composed of 15 percent cement, 30 percent sand, and 55 percent gravel. If a mixture of A and B contains 25 percent cement, what percent of the weight of the mixture is A?

A 20%
B 33 1/3%
C 45%
D 50%
E 75%
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by beat_gmat_09 » Tue Nov 30, 2010 6:06 am
Pick A - 100 units, B - 100 units.
35 units C from A 15 from B total - 50 units makes 25% of total (200)
A is 50% of Total (200)
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by diebeatsthegmat » Tue Nov 30, 2010 6:19 am
Mogorosi wrote:Concrete mixture A is composed of 35 percent cement, 40 percent sand, and 25 percent gravel; concrete mixture B is composed of 15 percent cement, 30 percent sand, and 55 percent gravel. If a mixture of A and B contains 25 percent cement, what percent of the weight of the mixture is A?

A 20%
B 33 1/3%
C 45%
D 50%
E 75%
A+B=25 percent cement
we can write the equation:
35A+15B=25(A+B)
so 10 A=10 B or A=B
so A/A+B=A/2A=1/2=0.5 or 50%
am i right?
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by GMATGuruNY » Tue Nov 30, 2010 6:39 am
Mogorosi wrote:Concrete mixture A is composed of 35 percent cement, 40 percent sand, and 25 percent gravel; concrete mixture B is composed of 15 percent cement, 30 percent sand, and 55 percent gravel. If a mixture of A and B contains 25 percent cement, what percent of the weight of the mixture is A?

A 20%
B 33 1/3%
C 45%
D 50%
E 75%
No complex math is needed here. Since the resulting percentage (25%) is halfway between the starting percentages (35% and 15%), we'll need equal amounts of A and B. Thus, 50% of the resulting mixture will be A.

The correct answer is D.
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