Original 45 new 30 what is the percent of decrease?

It is important to closely check all the data that are given in the question. Those data are important for finding the result.
Original = 45 
New after decrease = 30
Quantity of decrease = 45 - 30
                                   = 15
Percentage of decrease = (15/45) * 100
                                        = 100/3
                                        = 33.33%
So we get 33.33% as the percentage of decrease. I hope the procedure is clear enough for you to understand. This procedure will help you to solve this type of problems in future with absolute ease.