# How do you list all the prime numbers between 80 and 90?

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You evaluate the 5 odd numbers from 81 to 89.

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In division testing you may skip

We don't have to test any further: **83 is prime**

We don't have to test any further: **89 is prime**

**Note** : We didn't have to test for the next prime 11, because if the number has a factor of 11 or higher, it also has a factor smaller than 11, and we would have found that. The highest prime you test for is smaller/equal to the square root of the number.

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To list the prime numbers between

All even numbers are divisible 2, they are not prime. apart from

This leaves all the odd numbers.

A number ending in

If the sum of the digits is divisible by

As factors are always in pairs we only need to consider factors less than the square roots.

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