Try thinking of it in terms on how many total human minutes are needed in the game. If a game is 40 minutes, and 5 people play at a time, there are 40 * 5 = 200 "man-minutes" needed per game. (In the real world, projects are often billed based on how many man hours are needed. A team of 5 people working for 2 hours requires 10 man hours).
Any ideas where to go from here? We now know a game requires 200 total minutes. Keep in mind that you are wanting each person to contribute equally towards the total.