There are three placement methods:
The first type: three books are stacked together to form a cuboid with a length of 20 cm, a thickness of 15 cm and a width of 15 cm.
Surface area = 20×15× 2+20×15× 2+15×15× 2 =1650.
The second type: three books are side by side to form a cuboid with a length of 45cm, a width of 20cm and a thickness of 5cm.
Surface area = 45× 20× 2+45× 5× 2+20× 5× 2 = 2450.
The third type: three books are arranged vertically together to form a cuboid with a length of 60 cm, a width of 15 cm and a thickness of 5 cm.
Surface area = 60×15× 2+60× 5× 2+15× 5× 2 = 2550.
The first method is the least!
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