What is Base 40 in college grading system?

There's no standard "Base 40" college grading system. Grading systems typically use a base of 100 (percentage), 4.0 (GPA), or letter grades (A, B, C, etc.). A base 40 system isn't commonly used and wouldn't be recognizable in a typical college setting. If you encountered this term, it's likely specific to a particular institution, course, or instructor and you'd need to consult that source for its meaning.