how many different ways can 2 students be seated in a row of 8 desks , so that there is always at least one empty desk between the students ?
"21 ways to seat the students : with two empty seats between 1 empty w / one student on the left most 1 empty . . . . right most two students can be interchanged 21 x 2 = 42 b"
permutation(subtract(8, const_1), 2)