# Permutation & Combination

**Question**

There are few letters, namely “GIIILK”.

- How many different ways the words can be arranged in a row?
- How many ways of arrangement that all three letters of “I” are together?

**Answer**

- 20
- 6

## Permutation Formula

n!/(n-r)!

This formula calculates the number of permutations of n things taken r at a time

## Combination Formula

n!/[(n-r)!r!]

This formula calculates the number of combinations of n things taken r at a time

## Number of permutations of n things when there are n1, n2, …, nk repeated items.

n!/(n1!n2!…nk!)

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