In an NGO, the daily average wage of 20 illiterate employees decreases from Rs. 25 to Rs. 10. As a result, the overall average daily wage of all employees (literate and illiterate together) decreases by Rs. 10. How many educated (literate) employees are working in the NGO?

Introduction / Context:
This question uses the idea of how changing the income of a subgroup affects the overall average. It is a classic average and total change problem, useful for understanding how a change in one part of a group influences the combined average of the entire group of people.

Given Data / Assumptions:

• Number of illiterate employees = 20.

• Their daily wage decreases from Rs. 25 to Rs. 10.

• So each illiterate employee loses Rs. 15 per day.

• The average daily wage of all employees (literate and illiterate together) decreases by Rs. 10.

• Wages of educated employees are assumed unchanged.

Concept / Approach:
When part of a group experiences a change, the overall average changes according to the total change divided by the total number of individuals. Here, we first compute the total daily loss in wages for the illiterate group. Then we relate that total loss to the given drop in the overall average to find how many total employees there are. Subtracting the known 20 illiterate employees will give the number of educated employees in the NGO.

Step-by-Step Solution:
Daily loss per illiterate employee = 25 − 10 = Rs. 15. Total loss for 20 illiterate employees = 20 * 15 = Rs. 300 per day. Let total number of employees (literate + illiterate) = N. Overall average wage decreases by Rs. 10, so total daily loss = 10 * N. Equate losses: 10 * N = 300. N = 300 / 10 = 30 employees in total. Number of educated employees = total employees − illiterate employees = 30 − 20 = 10.

Verification / Alternative check:
To verify, suppose there are 10 educated employees whose wages stay the same. The 20 illiterate employees together lose Rs. 300 per day. Spread over 30 employees, the average drop is 300 / 30 = Rs. 10 per person per day, which matches the given reduction in the overall average. This confirms that the count of educated employees must be 10 for the numbers to be consistent.

Why Other Options Are Wrong:
If there were 15 or 20 educated employees, the total number of employees would change and the average drop would no longer be exactly Rs. 10 when we divide the total loss of Rs. 300 by that number. The option "Data Insufficient" is incorrect because we have enough information: the number of illiterate employees, the wage change and the change in overall average fully determine the total number of employees.

Common Pitfalls:
One common error is to work only with averages and forget to translate them into total amounts. Another mistake is to think that the number of educated employees is directly related to the original average, when in fact the crucial link is the total loss and the new average. Always remember that average change = total change divided by total count, which is the key relationship in such problems.

Final Answer:
The number of educated employees working in the NGO is 10.