Acity has a summer carnival every year. two years ago the carnival sold a total of 15,000 tickets. last year it rained, so the carnival only sold 12,000 tickets. what is the percentage decrease in the number of tickets sold?

The actual decrease is 15000-12000, or 3000, tickets.

Divide this 3000 by the original 15000:

  3000         3
= =  0.2.  Multiplying this by 100%, we get 20%.
15000        15

"Decrease" signifies a negative quantity.
We could say either that the ticket sales decreased by 20%, or 
that the change was -20%.