How many liters of a 60% acid solution must be mixed with a 75% acid solution to get 20 L of a 72% solution?

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Wow chemistry problem! Fun! =) You can solve it like this: Amt*conc.+amt*conc.=mix amt*conc we do not know the amount of 60% acid so let that be x: x*0.60+(20-x)*0.75=20*0.72 and then solve for x. When simplified: 0.60x-.0.75x+15=14.4 Add the x's and substract 15 on both sides: -0.15x=-0.6 x=4 so plug it back to the equation you get 4L of the 60% solution and 16L of the 75% solution Please check out this video for more examples: Math > Algebra > Word Problems Using Systems of Equations > Percent Concentrations > Problem 1