Your question is Measure Exactly Four Gallons. Take a moment with it on the right.
Talk me through your thinking if you like. When you're confident, submit your answer and I'll grade it like a real screen (7/10 or better passes).
You have 2 containers. One that holds 5 gallons of water and one that holds 3 gallons of water. You have unlimited water. How can you measure out exactly 4 gallons of water?