Example:
CREATE OR REPLACE PACKAGE comm_package IS
g_comm_rate  NUMBER := 0.1; /* initialized to 10% */
   PROCEDURE reset_comm_rate(v_comm_rate  IN  NUMBER);
END comm_package;
SQL> run

CREATE OR REPLACE PACKAGE BODY comm_package IS
FUNCTION validate_comm_rate
   (v_comm_rate  IN  NUMBER)
   RETURN  BOOLEAN
IS
    v_max_comm_rate  NUMBER;
BEGIN
    SELECT MAX(comm / sal) INTO v_max_comm_rate FROM pub_emp;
    IF v_comm_rate > v_max_comm_rate THEN 
       RETURN (FALSE);
    ELSE  
    RETURN (TRUE);
    END IF;
END validate_comm_rate;

PROCEDURE reset_comm_rate(v_comm_rate  IN  NUMBER)
IS
  v_valid   BOOLEAN;
BEGIN
  v_valid :=validate_comm_rate(v_comm_rate);
  IF v_valid = TRUE THEN
     g_comm_rate := v_comm_rate;
  ELSE
     RAISE_APPLICATION_ERROR (-20210,'Invalid comm rate.');
  END IF;   
END reset_comm_rate;  
END comm_package;

SQL> run
SQL> execute comm_package.reset_comm_rate(.15);

SQL> execute comm_package.reset_comm_rate(10.00);
BEGIN comm_package.reset_comm_rate(10.00); END;

*
ERROR at line 1:
ORA-20210: Invalid comm rate.
ORA-06512: at "BENJAMIN.COMM_PACKAGE", line 24
ORA-06512: at line 1

Q1.
CREATE OR REPLACE PACKAGE explanation_package IS
    g_explanation s_inventory.out_of_stock_explanation%TYPE;

    PROCEDURE reset_explanation
           (v_explanation  IN   s_inventory.out_of_stock_explanation%TYPE);

END explanation_package;
SQL> run

CREATE OR REPLACE PACKAGE BODY explanation_package IS

FUNCTION validate_explanation
        (v_explanation  IN    s_inventory.out_of_stock_explanation%TYPE)
        RETURN          BOOLEAN
    IS
    BEGIN
        IF ( UPPER(v_explanation) LIKE '%INVENTORY ITEM%')
        OR ((UPPER(v_explanation) LIKE '%PRODUCT%') AND
           ( UPPER(v_explanation) LIKE '%WAREHOUSE%')) 
        THEN
            RETURN (TRUE);
        ELSE
            RETURN (FALSE);
        END IF;
    END validate_explanation;

    PROCEDURE reset_explanation
        (v_explanation   IN  s_inventory.out_of_stock_explanation%TYPE)
    IS
        v_valid  BOOLEAN;
    BEGIN
        v_valid := validate_explanation (v_explanation);
        IF v_valid = TRUE THEN
           g_explanation := v_explanation;
        ELSE
          RAISE_APPLICATION_ERROR (-20301,
              'The explanation is not worded correctly.');
	END IF;
    END reset_explanation;

BEGIN
    g_explanation := 'New inventory item entered by ' || USER;
END explanation_package;
SQL> run

SQL> execute explanation_package.reset_explanation('testing');
BEGIN explanation_package.reset_explanation('testing'); END;

*
ERROR at line 1:
ORA-20301: The explanation is not worded correctly.
ORA-06512: at "BENJAMIN.EXPLANATION_PACKAGE", line 27
ORA-06512: at line 1

SQL> execute explanation_package.reset_explanation('testing inventory item');

PL/SQL procedure successfully completed.

2

CREATE OR REPLACE PROCEDURE add_inventory
        (v_product_id           IN      s_inventory.product_id%TYPE,
        v_warehouse_id          IN      s_inventory.warehouse_id%TYPE,
        v_reorder_point         IN      s_inventory.reorder_point%TYPE,
        v_max_in_stock          IN      s_inventory.max_in_stock%TYPE)
IS
        c_amount_in_stock       CONSTANT
            s_inventory.amount_in_stock%TYPE := NULL;
        v_out_of_stock_explanation
            s_inventory.out_of_stock_explanation%TYPE;
        c_restock_date          CONSTANT
            s_inventory.restock_date%TYPE := NULL;

BEGIN
        v_out_of_stock_explanation := explanation_package.g_explanation;

        INSERT INTO s_inventory (product_id, warehouse_id,
                  amount_in_stock, reorder_point, max_in_stock,
                  out_of_stock_explanation, restock_date)
            VALUES (v_product_id, v_warehouse_id, c_amount_in_stock,
                    v_reorder_point, v_max_in_stock,
                    v_out_of_stock_explanation, c_restock_date);
        COMMIT;

END add_inventory;
SQL> run
SQL> execute add_inventory(10025,101,500,550);
SQL> select product_id,out_of_stock_explanation from s_inventory;