SOLUTION: How many gallons of a 2% salt solution must be mixed with 20 gallons of a 7% solution to obtain a 4% solution?

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Question 345769: How many gallons of a 2% salt solution must be mixed with 20 gallons of a 7% solution to obtain a 4% solution?
Answer by Fombitz(32388) About Me  (Show Source):
You can put this solution on YOUR website!
Let A be the amount of 2% salt solution.
2A%2B7%2820%29=4%28A%2B20%29
2A%2B140=4A%2B80
-2A=-60
A=30gallons