In my opinion, most of the students would have done great in maths in childhood (say 1st to 4th STD), not because they were studying well, it might be because the concepts were easy. But what happens...
Suppose there are 'n' number of different things. You would like to 'select' or 'arrange' all or some of those things in an order. In how many unique ways can you do so? Permutations and combinations is...