• Finding one quantity from the other. In a class with a ratio of boys to girls of $\Tblue{2}:\Tgreen{3}$, there are $\Tblue{12}$ boys. The number of girls is $\frac{\Tblue{12}}{\Tblue{2}}\times\Tgreen{3}= 6\times\Tgreen{3} = \Tgreen{18}.$
• Splitting quantities. There are $\Torange{45}$ pupils in the class. The proportion of boys is $\displaystyle\frac{\Tblue{2}}{\Tblue{2}+\Tgreen{3}} = \frac{\Tblue{2}}{\Torange{5}}$. The number of boys is $\frac{\Torange{45}}{\Torange{5}}\times\Tblue{2} = 9\times\Tblue{2} = \Tblue{18}.$
• Ratio with more than two numbers. A sum of $\Torange{20}$ euros is to be split between John, Mark and Paul in a ratio $\Tblue{2}:\Tgreen{7}:\Tviolet{1}$. They will receive in euros \begin{align*} &\frac{\Torange{20}}{\Tblue{2}+\Tgreen{7}+\Tviolet{1}}\times\Tblue{2} = \frac{\Torange{20}}{\Torange{10}}\times\Tblue{2} = 20\times\Tblue{2} =\Tblue{40},\\ &\quad\frac{\Torange{20}}{\Torange{10}}\times\Tgreen{7} = \Tgreen{140},\quad \frac{\Torange{20}}{\Torange{10}}\times\Tviolet{1} = \Tviolet{20}. \end{align*}