15% = 10% + 5%. So what you need to do is figure out 10% then add half (5%).
10% is very easy to calculate, you just move the decimal place one to the left. For example, if the bill is $25.00 then 10% would be $2.50.
Then to figure out 5%, just take half of that, which would be $1.25.
Now you have 10% which is $2.50 and 5% which is $1.25, add them together for a total of $3.75.
