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...

2(N - 0.2N -300 - 450 - 500) = 7100
- with a couple assumptions not explicitly stated.
N should be 4000. Check me on that.

For 1. the final unit is length cubed (volume) - not possible area
For 2. we don't know the magnitude of b; if b=2, - this is an area
For 3. the final unit is length squared - possible area!!
For 4. the final unit is length - not possible area
Let...