To determine the area that can be covered with 15 gallons of paint, knowing that 3 gallos can be used to paint 675ft², first determine the area that can be covered per gallon.
To do so, divide 675 by 3
Each gallon of paint covers an area of 225ft²
Next multiply the area per gallon by 15 to determine how much will 15 gallons of paint cover
With 15 gallons of paint you'll be able to cover an area of 3375ft²