How many square feet in 10 feet x 15.9 feet?

The floor is 100 square feet.
The ceiling is 100 square feet.
Each of the 4 walls is 150 square feet.

The total inside area of the room is (100 + 100 + 150 + 150 + 150 + 150) = 800 square feet for painting,
OR
700 square feet painted and 100 square feet of carpet or tile.

You'll have a few drops of paint left over, since the window(s) should not be painted.