Answer:
Question is incomplete.
Assuming it means to ask, what the number of tickets they sold the previous year and the next year, the answer is:
Tickets sold last year:
x * (1 - 10%) = 18,000
x * 90% = 18,000
x = 18,000 / 90%
= 20,000 tickets last year
Next year:
= 18,000 * ( 1 - 10%)
= 18,000 * 90%
= 16,200 tickets next year.
