Your not better than me at math.
In general, the tip amount is the cost times 0. xx and your total including tip, is the cost times 1. xx where xx = the percent you want to leave. If you want to leave a 20% tip, multiply the cost by 0.20 to get the tip amount or multiply the cost by 1.20 to get the total including tip.
In general, the tip amount is the cost times 0. xx and your total including tip, is the cost times 1. xx where xx = the percent you want to leave. If you want to leave a 20% tip, multiply the cost by 0.20 to get the tip amount or multiply the cost by 1.20 to get the total including tip.