He uses 3/16 gal for 1/3 square foot.
1/3 square foot is 1/3 of 1 square foot.
1 square foot is 3 times as much area as 1/3 square foot, so he must use 3 times as much water. We need to multiply 3 * 3/16.
3 * 3/16 = 3/1 * 3/16 = 9/16
Answer: He uses 9/16 gallon.