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Let x = the amount of water (in liters), and y = the amount of the 70% acide solution.
First equation is for the total volume of the solution
x + y = 5 liters. (1)
Second equation is for the final mixture concentration
= 0.28 (2) (concentration is the ratio of the pure acid volume to the total liquid volume)
From equation (2) you get y = = 2.
Then from equation (1) the rest, 5-2 = 3 liters is the water volume.
Answer. 2 liters of water and 3 liters of the 70% solution to be mixed.
Check. = 0.28 = 28%. ! Correct !
Solved.