James Tanton has a method for using FCT instead of Perms/Combs. http://www.jamestanton.com/wp-content/uploads/2010/05/Pamphlet-on-Counting.pdf Perhaps the students are having problems because they are so similar?
Math 30-2 Staffroom -- A Safe Place to Share and Reflect
Permutations Type of Questions
Re: Permutations Type of Questions