Answer: 1 gallon
Explanation:
Hi there!
Using the equation (12*18)-(7*3), where 12*18 is the area of the wall, 7*3 is the area of the door not being painted, you are left with 196 square ft. of wall. further dividing 196/200 is 0.98, meaning that Mr. Smith will only need one gallon of paint for his wall.
[(12*18)-(7*3)]/200
(215-21)/200
196/200
0.98