Right to left is the way you do it
Consider this option:
1. if to re-write the condition, it is given: total time t=8 hours; upstream=downstream=6 miles; V_boat=4 m/h., V_current=V=?
2. note, that total time=time_upstream+time_downstream, where time_upstream=6miles/(V_boat-V) and time_downstream=6miles/(V_boat+V). Using this it is possible to make up and solve the following equation:

answer: √10
P.S. the roots of the equation are √10 and (-√10), only positive values is needed for V.
Answer:
1.2 liters
Step-by-step explanation:
1 bucket has a volume of 80 cL.
15 buckets have a volume of 15 * 80 cL = 120 cL
1 L = 100 cL
120 cL * (1 L)/(100 cL) = 1.2 L
Twice as long as little drop...