Take the cost of the bill and add the tip percentage of the bill. That is:
[tex]cost = bill + (bill \times\% \: tip)[/tex]
or
[tex]cost = bill(1 + \% \: tip)[/tex]
solve:
[tex]19.47 = 16.50(1 + \% \: tip)[/tex]
[tex] \frac{19.47}{16.50} = 1 + \% \: tip[/tex]
[tex]1.18 = 1 + \% \: tip[/tex]
[tex]\% \: tip = 0.18 = 18\%[/tex]
The tip was 18% of the bill.
