If X is 90% of Y, then what percent of X is Y ?

$$\eqalign{ & X = \frac{{90}}{{100}}Y \cr & \Rightarrow X = \frac{9}{{10}}Y \cr & \Rightarrow Y = \frac{{10}}{9}X \cr & \Rightarrow \frac{Y}{X} = \frac{{10}}{9} \cr} $$
∴ Required percentage :
$$\eqalign{ & = \left( {\frac{Y}{X} \times 100} \right)\% \cr & = \left( {\frac{{10}}{9} \times 100} \right)\% \cr & = 111\frac{1}{9}\% \cr} $$