Question

You want to leave a tip for your server that is 20% of the food costs. Find the tip if the meal cost is $133.1.5

Answer 1

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

Answer 2

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..