Decrease of 20%
1) Gathering the data:
1 dozen 12 roses ----- $15
4 dozens 48 roses $12/per dozen
2) Note that with the first price, four dozens would cost $60. But since the new price, we have $48 for those 4 dozens. So there was the following decrease:

So as we can see there was a decrease of 20% for the price of 4 dozens.