Math CentralQuandaries & Queries


Question from John:

I need to find the inside diameter of a cement silo with an outside circumference
118 ft and a wall thickness of 7 inches. thank you John.

Hi John,

The circumference of a circle is given by $\pi$ times the diameter. Suppose the inside diameter is $d$ feet.


Then, since the thickness of the wall is $7$ inches which is $\large \frac{7}{12}$ feet, the outside diameter is

\[d + 2 \times \frac{7}{12} \mbox{ feet.}\]

But the outside circumference is 118 feet and hence

\[\pi \left( d + 2 \times \frac{7}{12} \right) = 118\]

and thus

\[ d + 2 \times \frac{7}{12} = \frac{118}{\pi}.\]


\[d = \frac{118}{\pi} - 2 \times \frac{7}{12} = 36.4 \mbox{ feet.}\]


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