SOLUTION: I have 100 gallons of 70% solution. I need to mix it with a 90% solution to obtain a 75% solution. How much 90% do I need?

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Question 830185: I have 100 gallons of 70% solution. I need to mix it with a 90% solution to obtain a 75% solution. How much 90% do I need?
Found 2 solutions by TimothyLamb, amalm06:
Answer by TimothyLamb(4379) About Me  (Show Source):
You can put this solution on YOUR website!
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70*100 + 90x = 75(x + 100)
7000 + 90x = 75x + 7500
15x = 500
x = 500/15
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answer
mix 33.333 gallons of 90% solution
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Answer by amalm06(224) About Me  (Show Source):
You can put this solution on YOUR website!
Let A denote the 70% solution and B denote the 90% solution
Using the method of alligation,
0.75-0.70=0.05
0.90-0.75=0.15
So that A/B=0.15/0.05
E.g. there are 0.15 parts of A for every 0.05 parts of B
If there are 100 parts of A, then (0.15/0.05)=100/x
x=33.3 gallons of 90% solution (Answer)