3/5th of 20 liters of water is 20*(3/5) = <<20*(3/5)=12>>12 liters
5/6th of 18 liters of vinegar is 18*(5/6) = <<18*(5/6)=15>>15 liters
12 liters of water mixed with 15 liters of vinegar gives 12+15 = <<12+15=27>>27-liter mixture
#### 27