If a% of x is equal to b% of y, then of c% of y is what % of x ?

$$\eqalign{ & \frac{{ {ax} }}{{100}} = \frac{{ {by} }}{{100}} \cr & or,\,ax = by \cr & \Rightarrow y = \frac{{ {ax} }}{b} \cr & c\% \,{\text{of}}\,y = \frac{{ {cy} }}{{100}} \cr & or,\,\frac{{ {cy} }}{{100}} = \frac{{ {cax} }}{{100b}} \cr & {\text{Thus}}, \cr & c\% \,{\text{of}}\,y = \frac{{ca}}{b}\% \,{\text{of}}\,x \cr} $$