You need to measure out four gallons, but you only have a three-gallon jug and a five-gallon jug. How do you measure out four gallons exactly?

Keep the 3-gallon jug empty and fill the 5-gallon jug, then Fill the 3-gallon jug completely from the 5-gallon jug, leaving 2 gallons in the 5-gallon jug.

Then empty 3-gallon jug, and pour remaining 2 gallons In 5-gallon jug to smaller jug. now, this remaining 1 gallon capacity in a 3-gallon jug.

Again fill the 5-gallon jug completely then fill 1 gallon into a 3-gallon jug now you will get 4-gallon capacity in a 5-gallon jug.