The average monthly salary of 20 workers in an office is ₹ 45900. If the manager's salary is added, the average salary becomes ₹ 49200 per month.

Average monthly salary of 20 workers = Rs 45900

Sum of the monthly salaries of 20 workers $=\operatorname{Rs}(45900 \times 20)=$ Rs 918000

By adding the manager's monthly salary, we get:
Average salary = Rs 49200
Now,
Let the manager's monthly salary be Rs x.
Thus, we have:

$\frac{\text { Sum of the monthly salaries of } 20 \text { workers }+x}{21}=49200$

$\Rightarrow \frac{918000+x}{21}=49200$

$\Rightarrow 918000+x=1033200$

$\Rightarrow x=115200$

Therefore, the manager's monthly salary is Rs 115200.