How to call one function in another function ?

SQL> create or replace function fun_two( num in number)
  2  return number
  3  as
  4  val number;
  5  begin
  6  val:=num+500;
  7  return val;
  8  end;
  9  /

Function created.

SQL> create or replace function fun_one( num in number)
  2  return number
  3  as
  4  val number;
  5  begin
  6  val:=fun_two(num);
  7  return val;
  8  end;
  9  /

Function created.

SQL> select fun_one(10) from dual;

FUN_ONE(10)
-----------
        510

SQL> select fun_one(30) from dual;

FUN_ONE(30)
-----------
        530

***********************************

SQL> declare

  2       function bob(x number) return number is

  3       begin

  4         return x*x;

  5       end;

  6       function fred(x number) return number is

  7       begin

  8         return x+bob(x);

  9       end;

 10     begin

 11      dbms_output.put_line('Value: '||to_char(fred(5),'fm9999'));

 12    end;

 13  /

Value: 30

PL/SQL procedure successfully completed.

SQL>