please help me explain

The Applebee's buyer heard through others that Rubio Enterprises, a maker of restaurant supplies, recently invested $1.2 million in plant, property, and equipment. The investors for Rubio want an 18% return on their investment. Applebee's plans to buy 10,000 new electronic menus from Rubio and received a quotation of $500/unit. In order to make sure Rubio's returns 18% to their investors, Rubio's better make sure their overall costs do not exceed: $300 $300 Correct Answer $478.4 You Answered $500 $216,000