2B. The equation

$$
\frac{x-1}{x}+\frac{x-2}{x}+\ldots+\frac{1}{x}=\frac{3 x-20}{4}
$$

solve it in the set of natural numbers.
Solution. We can write the equation in the form

$$
\frac{1}{x}(1+2+3+\ldots+(x-3)+(x-2)+(x-1))=\frac{3 x-20}{4}
$$

Since we are looking for a solution in the set of natural numbers, the sum

$$
1+2+3+\ldots+(x-3)+(x-2)+(x-1)
$$

can be written in the form $1+2+3+\ldots+(x-3)+(x-2)+(x-1)=\frac{(x-1) x}{2}$, so according to this, we will write the equation in the form

$$
\frac{1}{x} \frac{(x-1) x}{2}=\frac{3 x-20}{4}
$$

According to this, $4 x-4=6 x-40, 2 x=36, x=18$.