| since i work in the food service industry.  i should chime in.
 When a discount a given you should tip on the ORIGINAL price.
 1) the same amount of work is required to prepare your "discounted" item.
 2) most servers pay a percentage of sales out to bussers/host staff/bartenders, usually 3% of sales.  when calculating sales the original tab is always used.  so you should always tip on the original amount or the server gets shafted because he just got a bad tip and he's also tipping out so he just made even less.  thats why tipping today should be atleast 18%.  15% for the server  and 3% for the support staff (bussers,host,bartenders)
 
 example.
 you eat out and it cost $100.  you have a discount for 50%.
 when the restaurant calculates how much food was sold, the amount of food sold is $100.  So you end up paying half and the restaurant pays for the other half.  The server gets tipped $18.  Tips out $3.  Ends up making $15 on the table.  INDUSTRY STANDARD.
 
 if the server gets tipped off the discount price... $9.  tips out $3.  He then ends up making $6 for the original $100 tab.
 
 correct tip, server makes $15
 incorrect tip, server makes $6.
 
 thats a big difference for a $100 tab.
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