To solve this problem you must apply the proccedure shown below:
1. You have that the total area is obtained by applying the formula for calculate the area of a rectangle and multiply it by the four walls:
[tex]At=(4)(15ft)(20ft)\\ A=1,200ft^{2}[/tex]
2. You know that each gallon of paint covers 100 ft², therefore, you can calculate the amount of gallons needed (which you can call [tex]x[/tex]), if you divide the total area of the walls by the area covered by each gallon:
[tex]x=\frac{1,200ft^{2}}{100ft^{2}}=12[/tex]
The answer is: 12 gallons.
