Question 1018568
The perimeter of the room is 2 x (22+15) = 74 feet. 

74 * 8.5 = 629 sq ft.

The door is 36 * 84 = 3024 sq inches

The windows are 30 * 42 * 2 = 2520 sq inches

Total 5544 sq inches to subtract. 144 sq inches are 1 sq ft, so 5544/144= 38.5 sq ft.

629 - 38.5 = 590.5 sq feet, or 3.03 cans. 

The problem states "approximately," so we don't know if they underestimate or overestimate.  It's too close to say whether the 4th can will be needed.