Mary and Matthew work in a donut shop. When Mary works alone overnighjt, she makes all of the donuts for the next day in 6 hours. When Matthew works alone overnight, he makes all of the donuts in 5 hours. On Friday night, Mary starts making the donuts by herself. After 2 hours, Matthew arrives and they begin making donuts together. From the time that Mary started, how long will it take to finish all of the donuts for the next dayt?