Something

select store_num, sum(total_amt - sales_tax)/count(*)
from orders_p use index (PRIMARY)
where ord_date>='20070101' and ord_date<='20070128'
and (ord_type not like '%C%') and (total_amt - sales_tax >= 2.00)
group by store_num

This took over 17 minutes to run?!

+--------+-------------+--------+--------+---------------+--------+---------+--------+--------+--------+
| id     | select_type | table  | type   | possible_keys | key    | key_len | ref    | rows   | Extra  |
+--------+-------------+--------+--------+---------------+--------+---------+--------+--------+--------+
| 1      | SIMPLE      | orders_p| index  | [NULL]        | PRIMARY| 21      | [NULL] | 6297645| Using where|
+--------+-------------+--------+--------+---------------+--------+---------+--------+--------+--------+

+----------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+
| Table    | Non_unique | Key_name | Seq_in_index | Column_name | Collation | Cardinality | Sub_part | Packed | Null | Index_type | Comment |
+----------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+
| orders_p |          0 | PRIMARY  |            1 | store_num   | A         |      [NULL] |   [NULL] | [NULL] |      | BTREE      |         |
| orders_p |          0 | PRIMARY  |            2 | ord_date    | A         |      [NULL] |   [NULL] | [NULL] |      | BTREE      |         |
| orders_p |          0 | PRIMARY  |            3 | ord_num     | A         |      [NULL] |   [NULL] | [NULL] |      | BTREE      |         |
| orders_p |          0 | PRIMARY  |            4 | ord_type    | A         |     6294251 |   [NULL] | [NULL] |      | BTREE      |         |
+----------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+