A 15% tip on $47.80 is $7.17. That's $47.80 × 0.15 = $7.17 exactly, so the total comes to $54.97 before any other charges.
If you'd rather round to something easier to hand over, $7 or $8 is common. Many people round up for good service and round down when service was poor. In the US, 15% is the usual baseline for standard table service, with 18–20% often used for very good service.
