$6.23
I am going to explain it two different ways.
31.15 x 0.20 = 6.23
31.15 +6.23 tip = 37.38 for you total bill including tip
An easier way my mom taught me for tipping in restaurants is you look at the bill $31.15 in this case move the decimal to the left one place and that give you 10%..double it and you have your 20% tip.
3.11 + 3.11 = 6.22
This way you don’t have to take a calculator out in front of anyone to calculate the tip.