Hi Dinesh,

The number $100 \times 99 \times 98 \cdot\cdot\cdot \times  2 \times 1$ is called 100 factorial and is usually written $100!$ For any positive integer $n, n!$ is defined by

\[n! = n \times (n-1) \times (n-2) \times \cdot\cdot\cdot \times 1.\]

There is no shortcut to calculate $n!$ For small numbers you can use Google. Try it. Type 10! into the Google search window.The numbers get large quite quickly. 15! has 13 digits and 100! has 158 digits.

To learn more I suggest you look at the Wikipedia page https://en.wikipedia.org/wiki/Factorial

Penny