Here let us take,
Total employee = 100

so customer service representatives = 75
programmers =15
managers =10

Now,

The avg salary of all the employees = 100,000

75 customer service representatives have avg salary of 60,000.
Difference in salary = 100,000 - 60,000 = 40,000

Let additional salary of manager = x

60,000 ....... 100,000........x

Therefore we can have the equation as -

75 * 40,000 = 10 * x (10 is the number of manager)

or x= 300,000

Therefore avg salary of managers = 100,000 + 300,000 = 400,000