Answer: 2 years
Explanation:
Years of existing of the firm=30 years
Number of associates= 300,
Number of Managers= 70;
Number of partners= 30;
Total number of workers=400
Number of years associates has been changed in last 30 year=30/5=6
Number of years managers has been changed in last 30 year=30/3=10
Number of times for partner=x
Number of years partners has been changed in last 30 year=30/x=15
15x=30
x=30/2
x=2 years