13.018. The depositor withdrew from his savings account at the savings bank first $1 / 4$ of his money, then $4 / 9$ of the remaining money and another 640 rubles. After this, he had $3 / 20$ of all his money left on the savings book. How large was the deposit?
Solution.

Let the deposit be $x$ rubles. Then the first remainder is $\frac{3 x}{4}$; the second remainder $-\frac{3 x}{4}-\frac{4}{9} \cdot \frac{3 x}{4}-640=\frac{3 x}{20}$. We have $\frac{3 x}{4}-\frac{x}{3}-\frac{3 x}{20}=640$, from which $x=2400$ (rubles).

Answer: 2400 rubles.