Updating one table with the data from another table with matching field

Kindly help with the following problem:
If there is a master table with employee info and another table called payroll consisting of 2 fields social.sec.num and salary.

Master_Table:     Name  Varchar2(20)
                          Age     Number(3)
                          Salary   Number (8)
                          Ssn      Number (10)

Payroll_Table      Ssn             Number (10)
                        Avg_Salary    Number (8)

Update the salary column of master table with avg_salary/12 for corresponding ssn. Use only SQL query. PL/SQL Block not allowed. Received on Wed Sep 17 1997 - 00:00:00 CDT