A baker wants to use all his flour to make bread. Working alone, he could finish the flour in 6 hours. With an assistant, the same could be done in 2 hours. The baker started working alone and, after some time, tired, he called his assistant and thus, after 150 minutes the flour ran out. For how many minutes did the baker work alone?
(a) 30
(b) 35
(c) 40
(d) 45
(e) 50