If x% of a is the same as y% of b, then z% of b is :

$$\eqalign{ & x\% {\text{ of }}a = y\% {\text{ of }}a \cr & \Rightarrow \frac{x}{{100}}a = \frac{y}{{100}}b \cr & \Rightarrow b = \left( {\frac{x}{y}} \right)a \cr & \therefore z\% {\text{ of }}b: \cr & = \left( {z\% {\text{ of }}\frac{x}{y}} \right)a \cr & = \left( {\frac{{xz}}{{y \times 100}}} \right)a \cr & = \left( {\frac{{xz}}{y}} \right)\% {\text{ of }}a \cr} $$