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

Answer: Option C

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