Wow chemistry problem! Fun! =)
You can solve it like this:
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