A vegetable vendor sold 1 kg of potatoes for Rs. 25 and earned 25% profit. In the evening, he started selling potatoes with only 10% profit. At what cost (in Rs.) per kg did he sell the potatoes in the evening?

Selling price of potatoes = 25
\[\begin{gathered} \begin{array}{*{20}{c}} {{\text{Cost price}}}&{{\text{Selling price}}} \\ \begin{gathered} 4 \hfill \\ \,\,\,\,\,\,\,\,\, \downarrow \times 5 \hfill \\ 20 \hfill \\ \end{gathered} &\begin{gathered} 5 \hfill \\ \,\,\,\,\,\,\,\,\, \downarrow \times 5 \hfill \\ 25 \hfill \\ \end{gathered} \end{array} \hfill \\ {\text{Profit }}\% = 10\% = \frac{1}{{10}} \hfill \\ \begin{array}{*{20}{c}} {{\text{Cost price}}}&{{\text{Selling price}}} \\ \begin{gathered} 10 \hfill \\ \,\,\,\,\,\,\,\,\, \downarrow \times 2 \hfill \\ 20 \hfill \\ \end{gathered} &\begin{gathered} 11 \hfill \\ \,\,\,\,\,\,\,\,\, \downarrow \times 2 \hfill \\ 22 \hfill \\ \end{gathered} \end{array} \hfill \\ \end{gathered} \]
Selling price of potatoes in evening = Rs. 22