Sentence Correction

This is from the Comparisons problem set of the Manhattan SC Bible 2003 edition:


Question:
Three times more students attended the prom this year than last year.


My version:
Three times as many students attended the prom this year than did last yr.


Ans Key:
Three times more students attended the prom this year than did last year.


Is My version incorrect? If yes, they why?


Thanks!