# Question #dec8d

##### 2 Answers

#### Explanation:

A prime number has only 2 factors : 1 and itself.

One way to recognise the bigger prime numbers is that they are the ones which we don't use in the times tables higher than 1 times.

So you can immediately exclude all even numbers after 2 and all multiples of 5 after 5.

If you are not sure whether a number is prime or not, add its digits. If the answer is a multiple of 3, then the number is not prime because 3 is a factor.

Try to know all the prime numbers up to 100. There are 25 of them.

Be careful of: 51, 57, 87 and 91. They are NOT prime.

Using these hints, you should be able to get the prime numbers between 11 and 31 as:

It is a good idea if you can to memorise some of the prime number between 1 and 100. Note that 1 and 100 are not prime numbers. If you do a internet search (images) you will soon find a list of prime numbers that you can save or print off.