For problems regarding any percentages like this for a total sum, you want to multiple the percentage as a decimal (in this case, 15% becomes .15) by the total number ($15.50). The number you get (2.325 or $2.33) is then added or subtracted by the total number. If it is a tip, you then add the number to find out the total amount. $15.50 + $2.33 = $17.83. If it were to ask for a discount, then you would subtract it.
15.50 * .15
15.50 + 2.325 (round to 2.33)
$17.83 is your answer
Hope this helped I can break it down more if you need it.
