Anu and Siri together can complete a piece of work in $20\text{ days}$. Anu alone can complete it in $40\text{ days}$. In how many days can Siri alone complete the work?

Let the one-day work capacity of Anu be $A$ and Siri be $S$.
According to the question, together they complete the work in $20\text{ days}$:

$$A+S=\frac{1}{20}$$

Anu alone can complete the work in $40\text{ days}$:

$$A=\frac{1}{40}$$

Substituting the value of $A$ into the first equation:

$$\frac{1}{40}+S=\frac{1}{20}$$

$$S=\frac{1}{20}-\frac{1}{40}$$

$$S=\frac{2-1}{40}=\frac{1}{40}$$

Thus, Siri alone will take $\frac{1}{1/40}=40\text{ days}$ to complete the work.