I'm studying Permutation & Combination those days and I've got well understanding the whole chapter but those word-problems related to it can't got them well, not even understand any of them. for an example :
1) $X=\{x : x \in Z , -2 \leq x \leq 5 \} , K= \{ (a,b): a,b \in X, a \neq b \}$
Find the number of elements of $K$.
2) If $X=\{3,4,5,6,7\}$, find without repeating any digit, each of the following:
(a) how many $5$-digit numbers can be formed from the elements of $X$?
(b) how many $5$- digit numbers can be formed from the elements of $X$ such that the unit digit is neither $4$ nor $5$?
(c) how many $5$-digit numbers can be formed from the elements of $X$ such that the unit digit is not $4$ and the tens digit is not $5$?
3) In how many ways can each of the following choice be done:
(a) Drawing $2$ playing cards from a pack of $52$ playing cards.
(b) forming football team ($11$ players) from $15$ players.
(c) forming a committee of $3$ Men and $2$ women from among $7$ men and $5$ women.
(d) Distribution of $8$ prizes equally among $4$ persons.
Plus i can't determine which problem to use Permutation & which to use Combination !! any help please !