Number of surface = 5
Surface 1: 8 x 5 = 40 . [The left side]
Surface 2: 15 x 5 = 75 . [The bottom side]
Surface 3: 17 x 5 = 85 . [The slanted side]
[Surface 4 and 5 are triangles and they have the same surface area]
Surface 4 : 1/2 x 15 x 8 = 60
Surface 5 : 60
Total : 40 + 75 + 85 + 60 + 60 = 320 square units