To start, we need to find the surface are of the deck we are trying to stain. If we were to draw a rectangle to represent the deck, we would put dimensions of 22 ft x 11 ft.
SA = length x width
SA = (22) x (11) = 242 ft2
Since the deck needs to be covered twice, we need to find the total surface area to be covered (aka double the surface area)
Total surface area to be covered = SA x 2
= 242 x2 = 484 ft2
We know each gallon of stain can cover 400 ft2 of surface area, which is close to what we need, but not quite enough.
To find out how many gallons we need to use, we would divide the total surface area to be stained by the number of square feet in one gallon. (See below)
(Surface area we need covered) / (coverage of one gallon)
(484) / (400) = 1.21 gallons
Now, since we cannot buy that little bit extra we need, we have to buy an entire second gallon, and have some left over.
Final Answer: 2 Gallons of stain must be purchased to stain the deck of specified dimensions.