Say I buy a $10 Groupon that lets me buy $20 worth of food at a restaurant. I go to the restaurant and order items that add up precisely to $20 (based on the menu price). If the state sales tax is 6% (specifically in Michigan if that matters), then how much in sales taxes should I pay (if any)?

$0? $0.60? $1.20 (6% on the full $20 value)?

It's quite possible that there simply isn't any legal clarity on this issue, as I've been charged varying amounts ($0 and $1.20 being the two most common possibilities in the above scenario).