Task 1. a) Fill in the cells with four different digits so that the product of the fractions equals $\frac{20}{21}$:

$$
\frac{\square}{\square} \times \frac{\square}{\square}=\frac{20}{21}
$$

Solve this problem for the other three arithmetic operations:

b) division

c) subtraction;

d) addition.

$[4$ points] (A. V. Shapovalov)
Answer.

$$
\frac{4}{3} \times \frac{5}{7}=\frac{20}{21}, \quad \frac{4}{3}: \frac{7}{5}=\frac{20}{21}, \quad \frac{9}{7}-\frac{1}{3}=\frac{20}{21}, \quad \frac{4}{6}+\frac{2}{7}=\frac{20}{21}
$$

There are other examples. Interestingly, in the last problem, it is not possible to use irreducible fractions.