Xavier and Ying are partners in a course project. Xavier is the project leader and is the first to decide how many hours, x, to put into the project. After observing the amount of time that Xavier contributes, Ying decides how many hours, y, to devote. The mark obtained by the project is 10(x + y) 0.5. Ying’s utility function is U = 10(x + y) 0.5 – y. How many hours does Ying work if Xavier works 15 hours? If Xavier does not want Ying to work on the project at all, how many hours should he work?