Math Circles Prime Time Worksheet With Answers - Grade 6 - University Of Waterloo - 2015 Page 3
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10Finding Primes
One (tedious) method of finding primes is called the Sieve of Eratosthenes:
1. Cross off 1 since it is not prime.
2. Find the first prime number (2) and circle it.
3. Go through the rest of the chart and cross off all of the multiples of two.
4. Go back to the beginning of the chart and find the first number that is not crossed off.
Circle it, it is prime. Then go through the rest of the chart, crossing off the multiples
of this number.
5. Repeat step 4, until every number in the chart is either circled or crossed off.
6. The numbers that are circled are the prime numbers.
Try it out:
Use this method to find all the prime numbers from 1 to 100. There should be 25 of them.
2, 3, 5, 7, 11, 13, 17, 19, 23, 29, 31, 37, 41, 43, 47, 53, 59, 61, 67, 71, 73, 79, 83, 89, 97
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