Answer:
The surface area of the lower rectangle is 20cm x 4cm= 80 square cm
the surface area of the two triangles is 12 square centimetres
the surface area of one of the top rectangular faces is 20×5= 100 square centimetres
the surface area of the other is 20×3= 60 square centimetres
80+12+60+100=252 square centimetres is the surface area of the polyhydron