Rahul's salary is first increased by $10\%$ and then again by $20\%$. But due to COVID-19, the salary is decreased by $25\%$. By what percent does the initial salary change?

Let's assume Rahul's initial salary is $\text{Rs. }100$.
1. First increment of $10\%$:

$${\text{Salary}}_1=100\times \left(1+\frac{10}{100}\right)=100\times 1.1=\text{Rs. }110$$

2. Second increment of $20\%$:

$${\text{Salary}}_2=110\times \left(1+\frac{20}{100}\right)=110\times 1.2=\text{Rs. }132$$

3. Decrement of $25\%$ due to COVID-19:

$$\text{Final Salary}=132\times \left(1-\frac{25}{100}\right)=132\times 0.75=132\times \frac{3}{4}=33\times 3=\text{Rs. }99$$

Change from initial salary:

$$\text{Net Change}=\text{Final Salary}-\text{Initial Salary}=99-100=-1$$

Since the result is negative, it indicates a decrease. The percentage change is:

$$\left(\frac{-1}{100}\right)\times 100\%=1\%\text{ decreased}$$