Let's say that you have 100 sophomores. 60 of them have chosen a liberal studies major. 24 of them have chosen a social history major and that leaves 16 undecided.
Now if 55 of all sophomores have chosen psychology as a course and who also have liberal studies as their major, the maximum number would, of course, be 55. The minimum number would be 55 minus 16 minus 24 or 15.
Therefore, the minimum percent of all sophomores who have chosen psychology as a course and also have liberal studies as their major subject is 15%.