The surface area of the region he will paint is 900 sq.ft.
Explanation:
Cubical room measures - 15 ft
Chaz want to paint only the walls of room
Surface of painting region means Lateral surface area (excluding top and bottom)
Lateral surface area of cube =
⇒ 4 * 15 *15
⇒ 900 sq.ft
The surface area of the region he will paint is 900 sq.ft.