Introduction

Factorial of a number n: n!

Fundamental Principle of counting

AND is multiplication (*) and OR is addition (+).

Concept of filling vacant spaces where the vacancy can be filled with and without repetation.

Whenever in counting problems constraints are given then we first aim to remove constraint then do other things.

Rank of a word in dictionary

Power of Primes in n!

Permutation and Combination

Combination is selection.

• nCr = n! / (r! * (n-r)!)

Permutation is selection then arrangement.

• nPr = n! / r!

One don't need to cramer this Permutation formulla because nPr = nCr * r! so Permutation is selection then arrangement.

Beggar-Coin Principle / Ball-Stick Method

Distribution of alike objects

Derangement