Prime and Composite Numbers

Charlotte Goldwater Kenwood Academy
5015 S. Blackstone
Chicago, Illinois 60645


The main objective is to have a systematic method of deciding if a number
is prime or not. This project is geared to fifth through ninth grades.

Materials Needed:

Large translucent container filled with water
Roll of masking tape
Any permanent marker
One or two vis-a-vis non-permanent markers
Three charts of numbers 1-100 on transparencies
A classroom set of the same chart on paper
A classroom set of an empty 10x10 grid
An overhead projector


The mini-teach is directed toward fifth through ninth grades. You give
students the definitions of prime and composite numbers. You allow students
systematically to go through the numbers crossing out or eliminating multiples
of two, multiples of three, multiples of five, multiples of seven, etc. You
continue until you reach the square root of the last number you are testing.
When you are done, the remaining numbers are prime. Have students make a list
of the numbers. For effect cut out the numbers from 1 to 100 from an acetate
copy of a Sieve of Eratosthenes. Put them on the overhead projector. Take a
pencil with a loop of masking tape and systematically remove the composite
numbers. The remaining numbers are primes.
Take fifty to one hundred pieces of masking tape, each about one inch in
size. Fold them in half so that the sticky side is together. In half of them,
put a penny. With your permanent marker, write the prime numbers on the pieces
of tape that have no pennies. Write the composite numbers on the pieces of tape
that have pennies inside them. Throw these numbers into a container of water.
The composite numbers will sink, and the prime numbers will float. You can have
the number 1 off to the side.
Take an empty 10x10 grid. In row six column five, place the number 11.
Then begin wrapping the numbers 12 through 110 around 11 in a clockwise fashion.
You will find many primes on the right diagonal of the line of numbers.
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