certainthings

I actually do 10% in my head and double it. So, say the bill is $6.50. 10% of that would be $.65, so double that would be $1.30, which is 20% of $6.50. And the trick to figuring out 10% easily is just to move the decimal point over to the left one digit. So 10% of 54.22 would be 5.42, and so on and so forth. I'm really terrible at math, but I've found this to be a pretty reliable method for me.