In a study relating college grade point average to time spent in various activities, you distribute a survey to several students. The students are asked how many hours they spend each week in four activities: studying, sleeping, working, and leisure. Any activity is put into one of the four categories, so that for each student, the sum of hours in the four activities must be 168. (a) In the model GP Ai = ?0 + ?1studyi + ?2sleepi + ?3worki + ?4leisurei + ?i does it make sense to hold sleep, work, and leisure fixed, while changing study? (b) Which standard assumption for OLS multiple regression does this model violate? (c) How could you reformulate the model so that its parameters have a useful interpretation and it satisfies this assumption?