The total salary amount = $15 \times 600 = 9000$
The salary of the exiting manager = 720.
Therefore, the salary of 12 workers and the remaining 2 managers:
When a new manager joins, the new average salary drops to Rs.580 for the total team of 15 of them.
The total salary for the 15 people i.e., 12 workers, 2 old managers and 1 new manager = $580\times 15=8700$
Therefore, the salary of the new manager is $9000-8700 = 300$ less than that of the old manager who left the company, which is equal to $720-300 = 420$.
Another alternate method of doing the problem is as follows:
The average salary dropped by Rs.20 for 15 of them. Therefore, the overall salary has dropped by $15\times 20=300$.
Therefore, the new manager's salary should be Rs.300 less than that of the old manager =$720−300$=420.