Usually, letters such asa,b, orc represent known or set values; letters such as x,y, orzrepresent the unknowns, things that can change, or what you're solving for.

Other variables aren,which represents some unknown quantity or number, probably becausen is the first letter in number; xis often the variable you solve for, maybe because it's a letter of mystery.Candkare two of the more popular letters used for representing known amounts or constants. The letterCis used frequently in calculus and physics, and it's capitalized in those cases probably due to tradition.