Joseph's salary is reduced by 10% . In order to have his salary back to his original amount, it must be raised by = ?

$$\eqalign{ & {\text{Let}}\,{\text{income}} = 100 \cr & {\text{Reduce}}\,{\text{income}} = 90 \cr & \% {\text{Required}} = \frac{{10}}{{90}} \times 100 \cr & \,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\, = \frac{{100}}{9}\% \cr & \,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\, = 11\frac{1}{9}\% \cr} $$