5x3 is the two lenths of the room. it could also be 10x1.5 but it wouldn't be easily accesseable.
225 square feet
16' x 15' = 240 square feet.
Easy to do. Just multiply the two dimensions: 15 x 20 = 300 square feet.
20 x 15 = 300 square feet.
you find square feet by multiplying length X width so a room 15' X 17' would equal 255 sq ft.