having trouble understanding

Classy Crulseline oflers nightly dinner cruises departing from several cities on the eastern coast of the United States including Charleston, Baltimore, and Alexandria. Dinner cruise tickets sell for $60 per passenger. Classy Cruiseline's variable cost of providing the dinner is $30 per passenger, and the foced cost of operating the vessets (depreciation. salaries, docking fees, and other expenses) is $270,000 per month. The company's relevant range extends to 15,000 monthly passengers. If Classy Cruiseline sells an additional 600 tickets, by what amount will its operating income increase (or operating loss decrease)