Moderator-80–
1) Pour the seven-gallon jug into one of the five gallon jugs until the five-gallon jug is full.
2) Empty the seven-gallon jug into the other five gallon jug. You will now have an empty seven-gallon jug, a full five-gallon jug and a five-gallon jug containing two gallons.
3) Fill the now-empty seven-gallon jug with seawater.
4) Pour seawater from the seven-gallon jug into the five-gallon jug that contains two gallons of oil until the five-gallon jug is full of a water-oil mix. You will now have a seven-gallon jug containing four gallons of seawater, a five-gallon jug full of oil, and a five-gallon jug full of a water-oil mix.
6) Empty the seven-gallon jug containing four gallons of seawater into the now-empty five-gallon jug. You will now have an empty seven-gallon jug, a five-gallon jug full of oil and a five-gallon jug containing four gallons of seawater.
7) Pour oil from the five-gallon jug containing five gallons of oil into the five-gallon jug containing four gallons of seawater until the second jug is topped off with a five-gallon oil-seawater mix. The first jug will now contain four gallons of oil.