- Education and Science
How to Memorize Prime Numbers
A prime number is a positive number that can only be evenly divided by itself and 1.It may be useful to memorize the 25 prime numbers between 1 and 100, especially if you are going to take a standardized test like the GMAT.
Sure, there are ways to find out if a number is prime or not mathematically instead of memorization, but I wanted to memorize at least some prime numbers so I can factor numbers faster and recognize at least the first prime numbers.
Please note: I'm not a teacher, but an individual who is embracing math and sharing things that have helped me.
Recognizing Prime Numbers
Don't forget: Prime Numbers over 5 end in 1, 3, 7, or 9.
The below hints for recognizing prime numbers are pretty obvious, but it helps to remember that prime numbers over 5 will end in only 4 different numbers.
- 2 is the only even prime number, because other even numbers are divisible by 2.
- 5 is the only prime number that ends in a 5, because all other numbers ending in 5 are divisible by 5.
- All prime numbers bigger than 5 end in a 1, 3, 7, or 9, but just because it ends in one of those numbers doesn't mean it's prime! It just means you won't find prime numbers over 5 ending in 0, 2, 4, 5, 6, or 8. That kind of narrows it down a bit!
The 25 Prime Numbers Between 1 and 100
It may be hard to just memorize a list of numbers, but here they are if you want to memorize them. I made up a story (below) that I used to memorize the prime numbers up to 100.
Make up a Story to Memorize Prime Numbers
It may help if you make up a story with the prime numbers between 1 and 100 in it in order to memorize them.
Here is the story I used to memorize the prime numbers. If this helps you, great but if it doesn't you can try making up your own story. I think the process of making up the story is what really helped me remember the prime numbers to 100. And of course you have to go over the story every so often in order to keep the prime numbers in your memory. I think it will be harder to make a story for memorizing prime numbers from 100 to 200 but maybe I will try that sometime.
I wrote the story below out sentence by sentence with the prime numbers in bold, and the prime numbers you should memorize from that sentence are also in parentheses after the end of the sentence.
The story (it's totally fictional and yes, I know it's ridiculous):
At 2:35, I went to 7-11 on Friday the 13th. (2, 3, 5, 7, 11, 13)
The cashier thought I looked 17, but I'm really 19 and he wouldn't let me buy a lottery ticket without an ID. (17, 19)
I bought 23 candy bars for my 29 year old sister who teaches a class of 31 students and needed some prizes to give those who get at least a B+ on a math test. (23, 29, 31)
To get a B+ on the math test, you need to get 37 out of 41 math problems correct. (37, 41)
The test ended at 43 after the hour, and 47% of the students got a candy bar and 53% did not. (43, 47, 53)
My sister gave the remaining candy bars to me and my 59 year old grandpa, and none to my 61 year old grandma who just lost 67 pounds. (59, 61, 67)
Many students scored between 71% and 73% and got a low "C" or "C-". (71, 73)
Her goal is for 79% of the class to get at least a "B" or "B+" which is 83% to 89% (79, 83, 89)
Forgot to mention, the highest score on the test was an A at 97%! (97)
Note: 7-11 (or more properly 7-Eleven) is a convenience store in case you didn't know.
Prime Numbers Chart
A chart like this that you can hang on the wall may help you in your memorization of prime numbers.
Memorize Prime Numbers by the number they end with....
Looking at the above chart gave me the idea that you could take the approach of memorizing prime numbers by grouping them by the numbers they end with. If you don't want to use the story method to memorize prime numbers, it seems a lot easier to memorize primes by grouping them this way than to memorize them in order. At least for me!
For example, besides the prime numbers 2 and 5 which are the only prime numbers that end in 2 and 5.....
You could then memorize the prime numbers ending in 1: 11, 31, 41, 61, 71
Next, you could memorize the prime numbers ending in 3: 3, 13, 23, 43, 53, 73, 83
Followed by memorizing primes ending in 7: 7, 17, 37, 47, 67, 97
Lastly, you can memorize primes ending in 9: 19, 29, 59, 79, 89
The disadvantage to this method is you might not remember the primes in order because you would know them by the number they end with instead. I'm not sure if that really matters for most people.
Another idea: Memorize the numbers ending in a specific number (1, 3, 7, and 9) between 1 and 100 that are NOT prime, so you would know everything else is prime ending with that number. And then put that with 2 and 5 being prime. I think this way might be a little more confusing though, but for numbers from 1 to 100 ending in 3, there are only three that aren't prime-- 33, 63, and 93. That at least is useful to keep in mind.
Prime Numbers Between 100 and 200
There are 21 prime numbers between 100 and 200
Once you have memorized the prime numbers between 1 and 100, perhaps you'll want to go for the prime numbers between 100 and 200? This will only be 21 more numbers to memorize!
I haven't memorized these yet, so don't have an entire ridiculous story for you, but you can make up your own. If you need help getting started, how about "There were only 101 life preservers for 103 people onboard. 107 people were supposed to come onboard the ship, but highway 109 closed down and a few people had to take Highway 113 instead but didn't make it on time and the boat left without them at 1:27 PM..."
Prime Number Lists
When you need to look up more prime numbers, or if you want to memorize more, check out the first 1,000 primes. If that isn't enough, check out the first 50 million primes which has some downloadable files listing prime numbers. These lists are from primes.utm.edu which also has a lot of information about prime numbers.