the ans is buy 5 vans and 15 cars. but I don't know how get these answer

6. Build a maximization or minimization from the following word problems. Graph and solve using the method of corners. A shuttle service is purchasing a new set of vehicles, vans and cars. They have secured a deal where they can purchase vans for $60,000 and cars for $20,000, and have a maximum of $600,000 to spend. They also can't purchase more than 20 vehicles, given the number of drivers they have on staff. If vans seat 4 passengers and cars seat 2, what number of vans and cars should they purchase to maximize the number of people they can carry