Willy likes playing football and computer games. Playing football for 20 minutes and Fortnite for 30 minutes makes him as satisfied as playing football for 50 minutes and Fortnite for 10 minutes. Suppose that he now plays Fortnite for 20 minutes, and his preferences are convex.

Given only this information about Willy's preferences, what is the theoretical maximum time he could spend playing football to be as well off as in the previous two situations? Please insert a numerical value only.