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.