She calculated that she could spend £12 pounds a day to pay someone to help with preparation. That way, the restaurant owner could spend the two hours working on the menus. This would only be worthwhile in a business sense if the new menus drew significantly more customers.
If the restaurant owner made £30 extra a day because of the new menus, then it would be well worth her time. In the first scenario, when she was doing the table preparation herself, she was not making any extra money. In the second scenario, the restaurant owner hired someone for the table preparations and spent her time working on the menu. Her extra profit was £30 extra a day minus the wage. Thus, in the second situation she earned an extra £18 a day.