Vivian : Cynthia = 2 : 9. Vivian paid $12.<br><br>
&bull; 2 units = $12 → 1 unit = $6<br>
&bull; Cynthia paid = 9 × $6 = $54<br>
&bull; Total cost = $12 + $54 = $66<br>
&bull; If split equally, each pays $66 ÷ 2 = $33<br>
&bull; Vivian must return $33 − $12 = $21 to Cynthia<br>
<br>
Final Answer: $21