The average monthly salary of 20 employees in an organisation is Rs. 1500. When the manager's salary is added, the average salary becomes Rs. 1600. What is the manager's monthly salary?

Introduction / Context:
This problem tests the ability to handle averages when one additional data point is included. It is a standard application of the average formula to determine the value of a new member in a group.

Given Data / Assumptions:
- Number of employees initially = 20.
- Average salary of these employees = Rs. 1500 per month.
- After adding the manager, total number of people = 21.
- New average salary = Rs. 1600 per month.
- We assume salaries are in rupees and are monthly.

Concept / Approach:
We obtain the total salary of the initial group and the total salary after including the manager using the relation:
Total salary = Average salary * Number of persons
The difference between the new total and old total is the manager's salary.

Step-by-Step Solution:
Step 1: Total salary of 20 employees = 1500 * 20 = Rs. 30000.Step 2: After adding the manager, number of persons = 21.Step 3: Total salary with manager = 1600 * 21 = Rs. 33600.Step 4: Manager's salary = New total - Old total = 33600 - 30000.Step 5: Manager's salary = Rs. 3600 per month.

Verification / Alternative check:
Divide the new total by 21 to confirm the new average: 33600 / 21 = 1600, which matches the given value. Also, if one of the 21 people earns 3600 and the remaining 20 earn an average of 1500, the combined average naturally increases to 1600.

Why Other Options Are Wrong:
- 3700, 3800 and 3900, when used as the manager's salary, would produce a new total salary higher than 33600 and hence a new average greater than 1600, which conflicts with the given condition.

Common Pitfalls:
- Forgetting to multiply the average by the number of people and instead directly adding 1600 to 1500, which is incorrect.
- Using 20 instead of 21 when computing the new total salary from the new average.

Final Answer:
The manager's monthly salary is Rs. 3600.