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%.