Last year, Cayman Corporation had sales of $30,000,000, total variable costs of $13,500,000, and total fixed costs of $5,000,000. In addition, they paid $3,000,000 in interest to bondholders. Cayman has a marginal tax rate of 35 percent. If Cayman's sales increase by 15%, what should be the increase in operating income?

the answer is 21.5% and need help figuring out how to work this problem