How many liters of a solvent that costs s80 per liter must be mixed with 6 l of a solvent that costs s25 per liter to make a solvent that costs s36 per liter?

1.5 liters
       Let's make an equation to represent the cost of the resulting solvent.
   80*A + 25*B = 36(A+B)
   Where
   A = amount of $80 per liter solvent
   B = amount of $25 per liter solvent
       So let's plug in the value 6 for B, then solve for A.
    80*A + 25*B = 36(A+B)
   80*A + 25*6 = 36(A+6)
   80*A + 150 = 36(A+6)
   80*A + 150 = 36A + 216
   44*A + 150 = 216
   44*A = 66
   A = 66/44 = 1.5 
So you need 1.5 liters of the $80 per liter solvent.