18% of $86.40 is about $15.55, so the total with tip comes to roughly $101.95. A quick way to check it: 10% is $8.64, half of that is $4.32, so 15% is $12.96, and adding another $2.59 (3%) gets you to $15.55.
If you'd rather round, $16 on the tip makes the math easier for the server and the bill. Tip amounts are a convention rather than a measured thing, so use whatever feels right for the service you got.
