To get a 20% tip, move the decimal point one place to the left and then double that number. $10.75 > $1.075 > $2.15. Then, add the $2.15 to the original price of $10.75 to get a total bill of $12.90. Hope this helps.
To find 10% divide by 10 so 1.07 or round to 1.08 if you like then double to get 20% so 2.14 or 2.16 so if the bill is 10.75 you would pay 12.89 with a 20% tip
