Exercise 7.5. Share of express among wash-fold bills per outlet.   [3 row(s)]
outlet | express | wash_fold_all | express_pct
-------+---------+---------------+------------
O01    | 155     | 751           | 20.6
O02    | 180     | 675           | 26.7
O03    | 70      | 347           | 20.2

