Question

A books thickness in 31 millimeters what is the thickness in centimeters

Answer 1

`\bf \begin{array}{ccllll} cm&mm\\ \textendash\textendash\textendash\textendash\textendash\textendash&\textendash\textendash\textendash\textendash\textendash\textendash\\ 1&10\\ x&31 \end{array}\implies \cfrac{1}{x}=\cfrac{10}{31}`

solve for "x"