Need two equations to solve this one.
Let x represent the liters 30% solution and y the liters 60% solution.
For liters of solution equation [ x+y=30 ]
For concentrations equation [0.3x+0.6y=0.4(30)]
Solve for the x and y using the two equations with two unknowns (just like a system of equations problem)
See if you can solve for and calculate the answer with this information.
Let me know if this helps.