If the cost price is 90% of the selling price, what is the profit percent ?

According to the question,
$$\eqalign{ & {\text{C}}{\text{.P}}{\text{. = }}\frac{{90}}{{100}}{\text{S}}{\text{.P}}{\text{.}} \cr & \frac{{{\text{C}}{\text{.P}}{\text{.}}}}{{{\text{S}}{\text{.P}}{\text{.}}}}{\text{ = }}\left. {\frac{9}{{10}}} \right\rangle {\text{1 unit profit}} \cr & {\text{Profit }}\% = \frac{1}{9} \times 100 \cr & \,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\, = 11\frac{1}{9}\% {\text{ }} \cr} $$