I can't math

Internet, this is EXACTLY how I feel every time I go to tip a waitperson at a restaurant:

I can grammar, though, so I wouldn't have made that particular gaffe.

However, it occurs to me that maybe I don't HAVE to math at all. Maybe if I just scribble an illegible tip amount and list a legible total, they can calculate the tip on their own, those people with a calculator or a cash register or whatever.

Can I? Huh? Can I?