Combinatorics
1. Correspondence⌗
Mapping one problem to a simpler and equal problem. Counting numbers between 14 and 103 is equal to counting numbers between 1 and 89.
Problem #1⌗
Count number of permutations $P$ of length $10$ such that $P=p_1,p_2,p_3,\cdots,p_{10}$ and $p_1>p_2>p_3>p_4>p_5$ and $p_5< p_6 < p_7 < p_8 < p_9 < p_10 $
Idea : $p_5$ is 1 obviously, now selecting any 4 numbers out of 9 will do as order is fixed.
Problem #2⌗
If a die is rolled 4 times. Find probability of the event where each of last 3 rolls are as large as the roll preceding it.
Idea : order is fixed again (nondecreasing), just count number of ways to distribute 4 positions among 6 candidates i.e. $^9C_4$. Useful idea to count number of sorted arrays for given constraints.
Derangements⌗
Number of derangements of a permutation are all those arrangements in which no element is at the same position as it is in original permutation. As an example derangements of (1,2,3) are (2,3,1) and (3,1,2).
General formula can always be derived by using inclusionexclusion principle, but it is not useful because calculating it will be O(n) and recurrence relation of derangement is more brief and intuitive and also takes O(n) time.

Deriving using Inclusionexclusion principle⌗
Let’s say $X_k$ be the set that contains element $k$ at position $k$.Then, $$D(n) = n!  X_1 \cup X_2 \cup X_3 \cup \cdots \cup X_n$$

Recurrence relation⌗
Obviously, $D(1) = 0$ and $D(2) = 1$. Let’s see what happens with $1$, we have $(n1)$ choices to choose a replacement for $1$ call that replacement $x$, now for any $n$ we have two options.
 Option 1 : Replace $x$ with $1$. Problem changes into finding $D(n2)$.
 Option 2 : Replace $x$ with some number other than $1$. So $1$ is not placed anywhere yet and other $n2$ numbers except $x$ are at their own position. So in total $n1$ numbers remain to be placed. Therefore, problem changes to $D(n1)$.
$$D(n) = (n1)( D(n1)+D(n2) )$$