Question 512137
Solution A
Amount = x 
Concentration =9% = 0.09
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Solution B
Amount = 2x 
Concentration =3% = 0.03
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Solution C
Amount = 15-3x 
Concentration =7% = 0.07
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Resultant Solution
Amount =15 
Concentration = 6%=0.06
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[Amount Solution A * Concentration A] + [Amount Solution B * Concentration of B]+ [Amount Solution B * Concentration of B] = Amount of Resultant * Concentration of resultant
(0.09)x+(0.03)2x+(0.07)(15-3x)=(0.06)(15)
0.09x+0.06x+1.05-0.21x=0.9
0.09x+0.06x-0.21x=0.9-1.05
0.15-0.21x=-0.15
-0.06x/0.06=-0.15/-0.06
x=2.5
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Solution A
Amount = x = 2.5 liters 
Concentration =9% = 0.09
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Solution B
Amount = 2x =2(2.5)= 5 liters
Concentration =3% = 0.03
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Solution C
Amount = 15-3x =15-3(2.5)=15-7.5=7.5 liters
Concentration =7% = 0.07
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Result
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He has to mix 5 liters of 3%, 7.5 liters of 7%, and 2.5 liters of 9% solutions to get 15 liters of a 6% solution