Four Gallons from the River

The Laboratory by the River

A river flows beside the laboratory. Using two barrels of 3 gallons and 5 gallons, how can you measure exactly 4 gallons of river water?

Goal: Measure exactly 4 gallons.

You can:

• Fill any barrel completely from the river
• Empty any barrel back into the river
• Pour from one barrel to another (until the source is empty or the target is full)