11.102. A copper blank in the form of a rectangular parallelepiped with dimensions $80 \times 20 \times 5 \mathrm{~cm}$ is rolled into a sheet with a thickness of 1 mm. Determine the area of this sheet.
## Solution.

The required area can be found using the formula $S=V / h$, where $V$ is the volume of the sheet, and $h$ is its thickness (the shape of the sheet does not matter). Since the volumes of both bodies are equal, then $V=80 \cdot 20 \cdot 5=8000$ (cm³). Therefore, $S=8000 / 0.1=80000 \mathrm{~cm}^{2}=8 \mathrm{~m}^{2}$.

Answer: $8 \mathrm{~m}^{2}$.