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(1)
Density of Fe = 7.87 g/mL
Volume of Fe = 16000 mL

Density = Mass / Volume

Mass of Fe
= (Density of Fe) × (Volume of Fe)
= (7.87 g/mL) × (16000 mL)
= 126000 g (to 3 sig. fig.)
= 126 kg


(2)
Volume
= (4.2 cm) × (3.8 cm) × (0.004 mm)
= (4.2 cm) × (3.8 cm) × (0.0004 cm)
= 4.2 × 3.8 × 0.0004 cm³
= 4.2 × 3.8 × 0.0004 mL


Density
= Mass / Volume
= (16.2 g) / (4.2 × 3.8 × 0.0004 mL)
= 2540 g/mL

The density calculated is too large.

If it is "0.004 m" instead of "0.004 mm", the answer becomes 2.54 g/mL which is reasonable.