Picture 3 vats. One vat has x liters of 18% acid. The second vat has y liters of 45% acid. The third vat is empty, but will contain 12 liters of the new mixture.
Build two equations. One equation is based on how many liters of the 18% solution is combined with the number of liters of the 45% solution to get 12 liters of the new mixture.
The second equation is simply the amount of acid in the 18% solution plus the amount of acid in the 45% solution equals the amount of acit in the 36% solution. Hint: The amount of acid in the 18% solution is 0.18x.
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