If you're not including tax, here's how you will do it:
15% can be converted to .15
you would then multiply .15 by 75.50
.15 * 75.50 = 11.325
since you can't give someone half of a penny, you must round up, getting 11.33
now you add 11.33 to 75.50
11.33 + 75.50 = 86.83
so, her bill would come out to be $86.83
Don't forget the "$" sign!
Hope I helped! Write a comment or message me if you have any more questions!
