The ratio of length to width of a rectangular sheet of paper is 5 : 3. If the width of the sheet is 18 cm, find its length? 

Given that ratio of length to width of a rectangular sheet is $5:3$

Given that the width of the sheet is $18 \text{cm}$

it implies that $3x=18\Rightarrow x=6$

Thereforem the length of the sheet is $5·6=\boxed{\mathbf{30}\mathbf{ }\mathbf{cm}}$