Sieve of Eratosthenes (finding prime numbers)
This grid is the sieve of Eratosthenes for numbers 1-100. To find the primes, I first cross out all multiples of 2 (except 2). Then, I cross out all multiples of 3 except 3. Then, all multiples of 5 except 5, and lastly all multiples of 7 except 7. Also, 1 is not prime so I cross that out – and then our list of primes is ready.
I also explain why we don't have to continue after 7... we actually don't have to cross out multiples of 11, 13, 17, etc. if all we're doing is numbers 1-100.
We also have an interactive sieve of Eratosthenes here at the MathMammoth.com website where you click on a number, and the program "sieves" the multiples of the number (leaving the number itself).
The second part of the lesson is here: Prime factorization.
See also
Least common multiple — video lesson
Math Mammoth Grade 6 curriculum