I found the answer using unit rate. Not sure what the difference between unit rate and unitizing is. Best I can tell is that unitizing is used mostly in grocery stores. How would this problem be solve with unitizing. Thanks

12/5=2.4 so 2.40dollars=unit rate

12dollars/5cds=2.40/1cd=unit rate

since it cost 2.40 dollars for 1cd, then 2.40 dollars X 9cds=21.60 dollars for 9cds ;)