Well to calculate area we use the equation: ( Height ) x ( Length )
12 x 120 = 1440 meter squares, this is the total area of the wall
Now since each paint gallon covers 150 feet square, we can simply divide 1440 by 150 :
1440 / 150 = 9.6 gallons
Now you can't really buy 9.6 gallons at Home Depot! So we will round to 10 total gallons of paint needed 
