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Prime Numbers

A prime number is a number that is divisible by 1 and itself only. Numbers that are notprime are called composite numbers. The smallest prime number is 2. It is also the onlyeven prime number. The prime numbers from 2 to 100 should be memorized. I suggestmemorizing the primes in sets of 10. For example, learn that 53 and 59 are the only primenumbers in the 50s.

List of Prime Numbers

2 3 5 7
11 13 17 19
23 29    
31 37    
41 43 47  
53 59    
61 67    
71 73 79  
83 89    

Questions involving prime numbers could ask for one specific prime number greater thanor less than another number, or how many primes are within a specific range.


What is the greatest prime number less than 25?

Knowing all the primes in the 20s (23, 29), the greatest prime less than 25 is 23.


How many primes are between 10 and 20?

Knowing all the primes in the 10s (11, 13, 17, 19), there are 4 prime numbers from 10 to 20.

Learning primes is important because many other tricks come from knowing prime numbers,including prime factorization, least common multiple, greatest common divisor, andothers.

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