13.164. The cost of 60 copies of the first volume and 75 copies of the second volume is 2700 rubles. In reality, only 2370 rubles were paid for all these books, as a discount was applied: 15% on the first volume and 10% on the second. Find the original price of these books.
Solution.

Let $x$ and $y$ rub. - the original price of the first and second volumes, respectively. According to the condition, $60 x + 75 y = 2700$. After the discount, the price of the first volume became $0.85 x$ rub., the second - $0.9 y$ rub. According to the condition, $60 \cdot 0.85 x + 75 \cdot 0.9 y = 2370$. Solving the system

$$
\left\{\begin{array}{l}
60 x + 75 y = 2700, \\
60 \cdot 0.85 x + 75 \cdot 0.9 y = 2370,
\end{array} \text { we find } x = 20 \text { rub., } y = 20\right. \text { rub. }
$$

Answer: 20 rub.