If the food cost is $133.15. you would take that and subtract 80%. leaves you with ($26.63)/ Your tip cost. I got taught this way..... since 100%- 80%=20%(your tip) you take, 133.15 - 80%($106.52) it gave me the total of the tip $26.63(20%) Hope this helps =) and wasn't too confusing..
this is what my parents make me do in restaurants,
tip is 20% 20%=10%+10%
finding 10% is easy, just move the decimal place one to the left then 20% is double of that so double it 133.15 move 1 down and double 13.315 double 36.63 tip is $36.63
