Problem: how many different ways can 2 students be seated in a row of 4 desks , so that there is always at least one empty desk between the students ?
Rationale: "total cases : 12 ( student one has 4 options and student two has three options , 4 x 3 = 12 ) non - favourable cases : 6 ( when two students sit together . students in desk 1 and desk 2 , in desk 2 and desk 3 , in desk 3 and desk 4 ) for each of these cases there are two possibilities because the positions can be interchanged . hence 2 x 3 = 6 . so favourable cases : 12 - 6 = 6 . answer : d"
annotated_formula: permutation(subtract(4, const_1), 2)