Currently, urban savings banks are known to pay a $3.5\%$ interest rate on deposits, but the interest is subject to a $10\%$ tax, meaning depositors only receive a $3.15\%$ interest rate. Let's show that this $3.15\%$ interest can be quickly calculated as follows: multiply the capital by the number of days; divide the result by 10000, and subtract one-eighth of that result. For example, how much interest does 2520 K accrue at $3.15\%$ over 320 days?

$$
\begin{array}{r}
(2520 \times 320): 10000=80.64 \\
80.64: 8=\underline{10.08} \underline{70.56}
\end{array}
$$

Thus, the interest is $70.56 \mathrm{~K}$.
We denote the interest by $k$, the capital by $t$, and the number of days by $n$. The correctness of our procedure is shown by the following equation:

$$
k=\frac{t \cdot n \cdot 3.15}{360 \cdot 100}=\frac{t n}{10000} \cdot \frac{315}{360}=\frac{t n}{10000}\left(1-\frac{45}{360}\right)=\frac{t n}{10000}\left(1-\frac{1}{8}\right)
$$

(Vilmos Erdös, Budapest.)