Question 989908
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N=amount of 90% solution.
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0.60(3 gal)+0.90(N)=0.81(N+3)
1.8gal+0.90N=0.81N+2.43gal
0.09N=0.63gal
N=7gal
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ANSWER You should mix 7 gallons of 90% solution with 3 gallons 
of 60% solution to get 10 gallons of 81% solution.
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CHECK:
0.60(3gal)+0.90(7gal)=0.81(10gal)
1.8gal+6.3gal=8.1gal
8.1gal=8.1gal