This is a fairly elementary question posed by you and the answer is one that you should be able to derive on your own. I have provided the same for your convenience.
5 cups of water are to be used by 6 students. So each student gets 1/6 of 5 cups or 5/6 cups each.
The fraction that each student gets is 5/6 cups.
If the problem stated that there were 6 cups of water, each student would receive 1 cup of water each. However, 6 students are attempting to evenly divide 5 cups of water so we now the students will receive slightly less than one full cup of water. The first thing to do is to divide 5 cups by 6. 5 divided by 6 equals .8333. Therefore, each student will receive .8333 cups or 5/6 of a cup of water. You can check this by multiplying 5/6 by 6 or adding 5/6+5/6+5/6+5/6+5/6+5/6. In either case, the answer is 5.