The surface area of one step is made up of two rectangles.
The flat step (tread) has an area of 10×25=250sq in and the riser has an area of 5×25=125 sq in. So the total surface area of one step is 250+125=375 sq in. This is the same as the area of a single rectangle 15×25. If you were carpeting the staircase you would multiply 375 by the number of steps to find out the total surface area of the staircase.