Berekening:
Opp. Voorkant = 4 cm x 5 cm = 20 cm²
Opp. Achterkant = 4 cm x 5 cm = 20 cm²
Opp. Bovenkant = 15 cm x 5 cm = 75 cm²
Opp. Onderkant = 15 cm x 5 cm = 75 cm²
Opp. Linkerkant = 15 cm x 4 cm = 60 cm²
Opp. Rechterkant = 15 cm x 4 cm = 60 cm²
Opp. Balk = 20 cm² + 20 cm² + 75 cm² + 75 cm² + 60 cm² + 60 cm² = 310 cm²
Of sneller...
Opp. Balk = (20 cm² + 75 cm² + 60 cm²) x 2 = 310 cm²