Next exams: Wednesday 2nd May, Thursday 3rd May, Wednesday 9th May, Thursday 17th May, Friday 18th May

In how many ways can you arrange the five numbers 1, 2, 3, 4, 5?

Think about the number of choices: five for the first choice, four for the second, and so on, so the number of ways is:

5 \times 4 \times 3 \times 2 \times 1 = 120

This calculation is called 5 factorial and is written 5!

In general for any positive integer n , n! = n(n-1)(n-2)... 1 \quad and 0! = 1.

Think about the number of choices: five for the first choice, four for the second, and so on, so the number of ways is:

5 \times 4 \times 3 \times 2 \times 1 = 120

This calculation is called 5 factorial and is written 5!

In general for any positive integer n , n! = n(n-1)(n-2)... 1 \quad and 0! = 1.

## Software/Applets used on this page

## Glossary

### factorial

n! = n(n-1)(n-2)(n-3)...3.2.1.

### integer

a positive or negative whole number.

### union

The union of two sets A and B is the set containing all the elements of A and B.