how many different ways can 2 students be seated in a row of 6 desks , so that there is always at least one empty desk between the students ?
"10 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 10 x 2 = 20 d"
permutation(subtract(6, const_1), 2)