Most colleges have a bare minimum to meet for math and science, let's just call them MATH 101 and SCI 101. The prerequisites for these should be done in high school and include advanced-level math and science courses to prepare students for them without a problem.
However, those courses aren't necessarily required for all students and all degrees.
Colleges know that some students are bad in these areas, as some enroll with GEDs, channel through ESL, or enroll by other means to get into their degree plans. To top it off, these areas of study aren't even applicable to many degrees, such as writing, for example. To address this, they offer corequisite courses — let's say MATH 111 and SCI 111, which, by the numbering, you would think is more difficult — as opposed to making students go straight into MATH 101 and SCI 101.
Corequisite courses like that are essentially just a simple brush-up of basic geometry, algebra, etc., in one course for math and basic sciences for the science. Also, they are easy enough to push students through the MATH and SCI requirements for their undergraduate degree.
The exception to this is if the students are going for a degree that requires advanced-level math and/or science (think STEM), the corequisite is essentially now a prerequisite, as without them, they would be unable to get into MATH 101 or SCI 101 in the first place. And oftentimes, the co/pre-requisite courses wouldn't count towards their math or science semester hours required for these degrees, but rather as an elective, if at all.
