Question 712276
Focus on the result first.  54 liters of mixture is actually 18 parts juice and 27 parts water.  Before the additions, it started as 1 part juice and 18 parts water.  You want to calculate the differences.  Avoid confusion:  as a lowest denomination 1 part is 1 liter.  

Before____________________After
1 part juice______________18 parts juice
18 parts water____________27 parts water


The question was, How many liters (parts) of juice were added?