Yang is a car salesperson at Honda dealership. He earns monthly salary of $2,400 and an $250 bonus for each vehicle he sells to a customer. Yang is planning a trip to Italy with his family. He needs at least $5,500 for the hotel and flight for the whole family. He plans to save his bonuses from selling cars for the next four months. If he seeks 7 cars in January, 5 cars in February and 3 cars in March, what is the minimum number of cars he must sell in April in order to have enough money for the trip? *only bonuses (not monthly salary)