## 98. Math Puzzle $7 / 73$

A plastic pipe has a density of $1.218 \mathrm{~g} / \mathrm{cm}^{3}$. The wall thickness is $2.0 \mathrm{~mm}$, the outer diameter $32 \mathrm{~mm}$, and the mass $0.8 \mathrm{~kg}$.

How long is the pipe?
Inner diameter $32.0 \, \text{mm} - 4.0 \, \text{mm} = 28.0 \, \text{mm}$

$F=\pi\left(R^{2}-r^{2}\right)=\pi\left(16^{2}-14^{2}\right)=1.884 \, \text{cm}^{2}$ Area of the annulus

$V=\frac{m}{\rho}=\frac{800}{1.218} \, \text{cm}^{3}=657 \, \text{cm}^{3}=$ Volume of the hollow cylinder

Length of the pipe: $1.884 \cdot x=657$, i.e., $x=3.49 \, \text{m}$