Explanation

Let's calculate the number of skilled employees (x) and unskilled employees (y):

Total salary of skilled employees = 520x

Total salary of unskilled employees = 420y

Total salary of all employees = 500(x + y)

Since the average salary is Rs. 500, we can set up the equation:

520x + 420y = 500(x + y)

Solving for x and y, we get:

x = 0.6(x + y)

So, the percentage of skilled employees is:

(0.6(x + y) / (x + y)) × 100 = 60%

Therefore, 60% of the employees in the firm are skilled.