Permutations and Combinations

**Factorial notation:**The notation*n*! represents the product of the first*n*natural numbers, i.e.,

*n*! =

*n*× (

*n*− 1) × (

*n*− 2) × …… × 5 × 4 × 3 × 2 × 1

0! = 1

**Example:**

**Fundamental Principle of Counting:**If an event occurs in*m*different ways, following which another event occurs in*n*different ways, then the total number of occurrence of the events in the given order is*m*×*n*. This is called the fundamental principle of counting.

**Example:**

Find the number of 5-letter words, with or without meaning, which can be formed out of the letters of the word MATHS, where the repetition of digits is not allowed.

…

**Solution:**…

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