Prime Number Checker

A prime number is a number which cannot be made by multiplying any other numbers together.

6 IS NOT a prime number because it can be made from 2 x 3.
7 IS a prime number. It cannot be made by multiplying any other numbers except 1 and itself..

Use this internet calculator to test if a number is a prime number.
(Do not enter a number with more than 5 digits - it may take the computer a long time to work out!)

Challenges:

Which is the biggest prime number below 100?

How many prime numbers are there between 1 and 20?